Rdr4evr, try reading through this forum.
http://www.gtaforums.com
It has a section dedicated to San Andreas and maybe they can fix some of the problems you are having. When I did my install on my computer, I defragmented before installed. So far I haven't had any major crashes. I think I had one crash that threw me to the desktop, but have not seen it again. I do have a problem with rain. When it rains, every other sound in the game turns off. Sometimes audio will completely disapear after the rain has finished. I'm hoping someone makes a mod where I can turn off the rain and the shimmering effect from heat. The shimmering effect from the heat or sun isn't a glitch, but it distorts my aim.
sadistikdreams, I had the same problem with Wrong side of the Tracks mission in Los Santos. I read on the gtaforums that you should ride on the outside of the right rail. Keep on the outside of that right rail and Big Smoke will be able to hit the gang members on the train.
Another helpful web page I found linked to the gtaforums was
http://www.gtasanandreas.net
Oh yea, here is a cool pinned item they have in the gtaforums. They have been finding and posting Secrets and Easter Eggs in the game. I've been looking for some of these as I play through the game.
http://www.gtaforums.com/index.php?showtopic=174800