i need help

[beefcakext]

My car is a 91 240sx and when i bought it the girl said that it would only run for half an hour then die and when it died the battery was dead and needed to be jumped. To me this sounded like an alternator. but since i took it home ive only jumped it once and it has never died on me but the rpm jumps around after a couple min of driving. it usuall jumps 3 to 400 rpms. When it does this i usually try and charge it. i havent replaced the alternator yet because now im doubting if that is wrong with it. If u have any suggestions they would help tramendously.

[Dattebayo]

You should take this one to the "online mechanic" forum.

But maybe one of us could help if you are more descriptive on how, when and under what circumstances the rpm's "jump" around.Also any other noises it makes would help too.

[beefcakext]

ok thanks man