extremely low idle - sometimes stalls

hey everyone. Ok im going to explain my problem, and whent he problem started happening. It all started one day after I changed my alternator. I did the alternator swap just fine, checked that the belt was tightened properly and everything. The charging system works great, but later that day i went to start my car and noticed that it idled REALLY low and then stalled. i started it again this time holding the gas so it wouldnt stall, for about 30 seconds, then I let go of the gas and it was fine. i drove away and i noticed after about 10 minutes when i pulled up to a red light, it started idling really low again then bouncing back up to normal idle, then dropping back to low idle and almost stalling. then i came home. the next day in the morning i went to start it, same thing, i had to hold the gas down for about half a minute until it would hold proper idle on its own. even after tightening my throttle cable it still does this. i dont get it! please help me! i have a 1999 GSR motor in my 91 teg.