About a month ago..after I wasn't using the device for an another month or so, only charged it when it got empty. Its software started showing weird symptoms like.. touch screen taps not registering.. getting stuck between desktops (didnt automatically move to the closest desktop anymore).. overall lag.. weird behaviour.. and then it went away.. came back every few days for 1-2 minutes or so. Today it hit me again, but this time it didn't go back normal.. it stayed like this.. totally unusable.. it took me 30 minutes to get the chance to read an sms which a friend sent me cause it didnt understand me pressing the conversations icon.. took out the battery, let it stay off for a while hoping it would make a difference - nothing. I haven't modded the system or overclocked, its pretty much vanilla with some free office-like apps and a couple of games. I don't want to reinstall the software, if there is no guarantee it will even help (maybe issue lies elsewhere?).

Any ideas?

There is a guarentee in that if you cannot get to the culprit app then a re-flash does in fact gurarantee the device is back to normal, just do a backup first of everything you need to keep.

Try de-installing from the last thing you installed.

uh oh.. somehow managed to uninstall latest apps and even some others too which I didn't need anymore, unfortunately it changed nothing, it's still crazy. Any "right" way to flash it?

FWIW: I was unsuccessful with flashing via my Ubuntu machine a couple of times, but it worked on the first attempt flashing from the Windows computer.

I've been having problems that are somewhat similar, lots of lag switching between open applications and switching desktops, the display being completely unresponsive at times and graphical gliches. In my case when I check "dmesg" I get lots of lines showing "HWRecoveryResetSGX: SGX Hardware Recovery triggered". You might want to check dmesg yourself to see if there are errors indicating a hardware problem.

In my case I think it is flaky hardware because I use my N900 a lot, it only started today and I haven't installed, uninstalled or even so much as updated anything for over a month. And if it persists I'll use rsync to clone this N900 to my spare, although I might try reverting to using the stock kernel first.

In any case, for you I would still recommend you doing a full flash and not installing anything, then if you still have the problem you can be pretty sure it really is hardware and perhaps send it back to Nokia if it is still in warranty.

Before flashing, you may want to install Backupmenu to create an image which will allow you restore you device exactly the way it was at the time of creating the image, although that image will only be of use if it turns out not to be a software problem and you get a replacement N900.

dmesg didnt give any errors, reflash didnt work either.. But I noticed the phone is responsive when navigated ONLY by using keyboard (obviously uncomfortable) .. touch screen failed? I haven't dropped it ever or let into contact with liquids.