Really low idleing and just shuts off at stop lights??

[whiteb]

i just bought a 91 240sx hatchback. whenever i start the car it sounds as if its struggling for it life, it idles around 600 rpm and will drop to abput 200 and start reving from 700 to 200 rpm back and fourth never even hitting 1000 rpm. never holds a steady idle speed, when i drive the car runs like a champ but idleing is a major problem shuts off on the freeway sometimes, all stock except for a borla exhaust, any suggestions!!!