I find that when I use any application that needs the keyboard my iPhone runs slower afterwards.
So i just went to Settings->Sounds and turned of the keyboard sounds and my iPhone is much more stable now.
Hope this helps.

Wow, not only did that suddenly make it dramatically faster, but truphone actually work (instead of immediately crashing)...
that's an interesting bug...
didn't solve the ipod video issue though (some vids won't play at all)

This was also noticeable back in the official 2.0 and 2.1 running on 1st gen iPhones.