800x600 < Agreed. On any computer I would always use this.
If he is familiar with q3 then it is the same issue with texture crap. Is it baseoa standard game and maps you are playing?
What server?
If I use 800 res and I have high detail up, the first thing I notice is that my weapons look much more detailed than the rest of the fov.
Playing on a server which has high detail maps is the main cause. The fps bouncing up and down is due to the mesh crap which fromhell mentioned. r_detailtextures 0 will improve this. r_lodbias 2 - ? I think this will help too.
It could be a server admin issue too
The admins have more control over the net settings than you do. If fact, you may not have any control despite what your settings tell you.