err ... I'm not aware of any mplayer upgrade - I've got SVNr30099-4.2.1 and i've never consciously upggraded mplayer.

There is nothing showing in the repos for mplayer, there's no TMO announce thread with a .deb or binary attached, and a google search of TMO didn't reveal any recent mplayer news either. Care to expand on what mplayer version you've obtained and from where?

fair enough, however just because something worked in the past doesn't stop it becoming unusable in future owing to server-side changes. Dropbox & Groove spring to mind as examples of such app's; BBC iplayer is a closer example of a video streaming service which used to work great on n900 but which is now very difficult to use owing to repeated changes at the BBC end (stream formats, bitrates and flash versions etc.).

Having said that, I've just tested tvcplayer again and it now seems to be working much better for me again, even at stock speeds. Using the "quality" preset:

600Mhz: occasional pauses/slowdowns but watchable unless it is live action/SFX stuff
700Mhz: noticably smoother, no significant pauses but still (less frequent) slowdowns
750Mhz: as good as performance when I originally installed tvcplayer
800Mhz: creamy smooth

The only changes at my end are that I recently changed the channel and position of my wireless access point to minimise overlap from the plethora of other AP's in the many neighbouring flats (wifi eye is your friend!).

I don't know if that has made the difference or whether something else has magically improved the performance of my n900 but there must be some reason. Maybe a server-side performance improvement? how many servers/admin staff do they have to host the streaming on what is a free, ad-supported service? (they may simply have tweaked load-balancing or whatever).

Anyway, you could see if messing with your AP helps you any. If not, you could PM arajantie. Thereafter, a speculative reflash may be worth a try.

Otherwise I'm afraid overclocking is the quickest, easiest, and possibly only practical way to get you back to where you want to be without external help. And, seriously, with the age of the n900, and the ~2 years worth of overclocking experience of hundereds of users on TMO, is it really such a scary prospect to push the CPU to 75%-80% of it's maximum speed for the odd half-hour's viewing pleasure :-) FWIW, I've watched probably 10-15 full 90 minute football games on my n900 running at 900Mhz (internet streams, played onto big TV's while I've been abroad or staying over at friends etc.) and my phone is still perfectly fine after 22 months.

Good luck!