car is surging

can anyone tell me why my car surges at idle.(idles up and down)it doesn't do it all the time,though it seems it is more frequent when car heats up.at times it will die.but if you take it out on road car runs as if it were a new car no problems only at idle in drive or park.also has a little miss in it at the same time as surging.i've also noticed that when car is running if you turn on heater or headlites(at idle)the dash lites will dim and car idle will change.had it checked and it is charging good at advance auto.(although when i removed the alternater the battery wire was might near burned into,but i did fix that part,but still surges at idle)but keep in mind it doesn't always surge,only about every 5-10 minutes,but doesn't do it accelerating on highway.any help would be appr. it has a 3.1 V6 F.I. 1989