Can I install extra linux stuff without having to reflash? Thank you.

What do you mean ?
Off course you can install if there is no other problem with the phone. But if your phone is acting up, rebooting draining your battery for now particular reason heating up without you actually using or recharging it then no. If you dont know what the problems are you fix it, if not the easiest way is to reflash both eMMC and firmware and start over installing your programs and scripts one at the time. This way you can exclude witch program started the problem the first time.

I don't recommend continuing to install hack tweek your phone if there is a problem. This often brick your phone. back trace your steps and try to think of what started the problems in the first place.

The beauty of the N900 is you can do anything with it. Anything goes as long as you know what you are doing. I haven't heard of anyone destroying or damage there phone by over-clocking it or changing voltage settings as long you have done your homework and sets any value to a "dangerous" setting.

Hope this answer your question.

No, my main issue is the update of the Media player. Everything was fine until this upgrade.

(With a risk of me sounding stupid) The stock media player in maemo or are you using another like KMPlayer ? Have you extra decoder pack installed ? Do you have the same problem as I described before ?
A little more info about your n900 and what you have installed like kernel power and if you have over clocked it and I will try to help you.

Thank you, I appreciate your help very much!

I have several players installed, but mostly uses the stock media player that came with N900 to play music and video.

I tried to install it, but kept getting an error message. So even though it did show in the list of installed apps, I deleted it.

I have installed the kernel power yesterday, as per your instructions, however, I did not reflash the phone. The findings: the phone became slightly faster. Some videos played well; but with some the phone hung again. Some of these either play on other players for a while, or don't.

Before I could play the vast majority of videos on the stock media player no problem. Now the phone either hangs or I get a weird message 'can't connect to the server', even though it's a video file.

I blame the recent stock Media Player update; before it everything was fantastic.

To OP:
As warned by titan and joerg_rw (power kernel and usb-otg respective developers), you should always disable AND say you did any clock risings before any calling for help about "strange behaviours" of the phone applications.
It is not yet seen, but is warned that this >>CAN<< cause permanent damage.

WAIT!!!! I must warn you that flashlight-EXTRA adds a "loop" in dbus calling that eats your battery (because it never let the processor stop).

I had this problem after reflash with firmware 1.3 with stock settings I then installed power kernel used same settings as pre reflash thinking this would help. It didn't. I then did it again and also flashed the eMMC. The phone worked flawlessly with firmware 1.2 with same configuration the problems started after update. Maybe I forgot something or there was a bug with the new firmware, I had this problem with 2 n900 .Then I asked for help.

Yes you are right, you should always do your homework considering the risks. I would not recommend this settings if I hadn't tried this my self. I had my phone at 1150 MHz using power kernel and titans settings for 3 months, but it got a little to unstable and rebooted to often. I have then used the same settings running it at 1000 MHz for over 6 month and never had any problems except the ones you get installing not fully tested software. My phone isn't damaged (yet) and I also warn people to do what I have done at there own risk.

You are right again, Thank you. This is something a haven't noticed. I have my charger in my car so it gets recharged when I go to work and home again. I also haven't had a battery logging program long enough to notice. Sorry for any problems I may have caused you

However I have the solution thanks to dh3x for them who already have installed it.
Just uninstall flashlight-extra wont fix it because the installer won't remove the "flashlight-extra" from /etc/event.d and /etc/init.d
You have to manually deleted these files.
I find this easier with Midnight commander (mc)

Uninstall flashlight-extra and then reboot.
Then start X Terminal write:
sudo gainroot
mc
Then go to /etc/event.d and delete the "flashlight-extra" file from that directory.
Then go to /etc/init.d and delete "flashlight-extra" file.
reboot.

I also still have problem playing video files if the format is not supported originally by the stock media player. If I try to play a format not supported I get the message "Media format not supported" then it wont play ANY video files. Only thing that seem to fix that again is a reboot.
I only use file formats that are supported and that works for me.
It sound to me that a complete reflash of firmware and eMMC is the way to go with your phone, I don't have enough knowledge of Maemo or Linux to manually set everything right when some program have run havoc with your device. Also it often is a ton of work finding faults and bugs, it's more time saving and less frustrating to just have a clean install and reinstall the programs you know work. As for me I just found out that the Flashlight-extra program had a bug that drains your battery. Luckily the fix was a easy one this time.

Sorry been on holiday. Will give it a try when I get a spare moment.

Big thanks in advance.

No problem.