engine problem please 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.

[steve s14]

if you have a volt meter, check the voltage of the battery with the engine running next time the idle starts to fluctuate. make sure the voltage is around 13v. sounds like it has a intermittent charging system problem. if the voltage is checked while the engine is running and the voltage reads below 12v, replace the alternator with a factory reman. do not get a pep boys or autozone special. although it's cheaper, you'll regret it when you find yourself replacing them all the time.

[beefcakext]

thanks man ill try that

[SimpleEnigma]

speaking of alternators....anyone know where a person could get a high current alternator...like a 95 or 105 amp unit? apologies for high jacking the thread.