can't find that '''sa-mp.cfg''
My Documents -> GTA San Andreas User Files -> SAMP -- sa-mp.cfg.
If it's not there, remove your GTA San Andreas User Files and join any server doesn't matter if it appears working or not, might as well start SP too, just to get those User Files fresh and back.
Also, remember to have SA-MP closed while doing this, also go to Processes Tab and shut down gta_sa.exe if it's running in the background. Just to make sure you're doing it right.
EDIT: Also, this could be a problem with connection between you (client) and RakNet/SA-MP (server).
If what I said doesn't help, and your internet and other games work online properly, you might as well post on SA-MP forums just to see what Devs, Testers and general public say.