When engine is warmed up the car begins hesitating

i have a 2000 nissan xterra and it started having a problem all of a sudden. i was driving around & when i was stopped at a stoplight it started idling irregularly, the rpm needle was going up & down then when i tried to go it hesitated - like it wasn't taking gas or something. it would kind of lurch & then pull back with the rpm needle going up & down accordingly. the next day it started up just fine, ran just fine - until the engine warmed up, then same thing, limping down the road - feeling like the car was going die - but never stalled. i have a friend looking at it & he says they've ruled out all the basic easy stuff. they thought it was a sensor, or electrical issue but still haven't pinpointed the problem. ANY IDEAS????? My 2002 Xterra V6 4x4 was running fine minus the AC stopped working a afew months ago when I stopped to get gas (gas light was on) I was leaving the station and it stalled while at idle then ran fine until I came to the next stop and it started to run rough then died and did this at every stop until I got home, it died in the drive way. It starts back up but then if not immediately given gas and moving it dies. Just a side note it is about 90 degrees and sunny out today. Any suggestions...I saw on other posting where people has said the throttle bottle spacer which we will pull and clean tonight