engine trouble

[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.

[The Mic]

Maybe more details?
Modified by S13GUY at 9:56 AM 2/3/2005

[beefcakext]

after i drive it around for a like 10 min the rpms start to fluctuate. Example if you were holding the rpms steady around 2grand without you doing anything they would die down to almost a thousand then go back up again.when i drive the engine is quiet no abnormal sounds just the purring of the exhaust. Umm the girl that i bought it from said that for her it would run for about a half an hour then die and the battery had to be jumped. Ive had it for 3 days now and it hasnt died on me once. I did charge the battery one time since i got it. umm thats all i can think of if u need more tell me

[midwestdrifter]

Try driving around with all the accessories on. If it dies then its the alternator, or the belts.

[Tech12]

Could be a timing issue as well.

[beefcakext]

ya it just died this morning but heres the thing. The lights will come n and the radio will work but when i turn the key nothen happens. Just silence.

[Tech12]

Check your grounds all along the engine bay, specifically the ones to the rear and right of the valve cover. Also, get a new battery and give the starter a couple of wacks with a broom handle.