problem with my idle!!!!

ok I have had this problem since my serpentine belt was replaced then tightened after driving for a couple days...

when i start my 89 XJ is sometimes idles really high and stays idling high for a long time, then it will drop down and idle low and sometimes the xj will just die while im driving!

I have cleaned out the AIC and the TPS but hasnt done anything to fix the problem?

does anyone have any suggestions on what i can do to fix the problem or how i can figure out whats wrong?

thanks