well ... firstly can i assure you that I'm using the driver "nvidia" rather than "nv" (or at least that's what my XF86Config-4 says!)
Secondly ... using xv is not required by mplayer. As mentioned in my first post, it's the only vo that's produced this effect. Unfortunately, xmms does not have the option of altering the driver ... and this continually causes problems.
x11, gl2 and other mplayer vo's work fine, and although things do slow down, it's not (usually) so much so that i'm unable to watch the media. However, every so often mplayer uses 100% of the 1Gz Athlon cycles, and the media becomes un-watchable.
The SDL driver has some unfortunate drawbacks with it ... the most obvious being that you are unable to use the keyboard or switch screens.
This isn't really a case of "need to fix", but a case of "isn't doing what it should be doing, and although there are work-arounds in some cases, it's VERY annoying"
I've a friend who has the same hardware as I do (with some extras) but only a 900MHz Athlon CPU and his mplayer seems perfectly stable. I can't see why there should be a difference in the stability, and it's left me truely perplexed!