I'm having similar problems to the same effect.
When I join any server, the game only runs at 1 or 2 fps for some reason. It used to play fine then one day it went like this. I reinstalled all of GTA:SA (including to the latest SA:MP) but then I downgraded to 0.3e but to no avail.
Thing is, when i start GTA:SA locally, it's still screwed so this probably isn't a server side problem, or anything to do with SA:MP for that matter.
Does anyone happen to know how to fix the problem? I haven't played in weeks :'(