major engine miss only when engine is hot

This is starting to p**** me off guys ;) Listen to this, only when the engine has been running for at least 15 minutes it start to miss a little then becomes major to a point that I have to stop and the first time got it towed to my mechanic... 1st time he flushes and cleans the throttle hoses I think (bare with me, give me glass and fire and I'ill make a work of art, but engines I know nothing). I leave with it and the next day it starts again... This time he say a loose wire on one spark plug and thinks that was the problem... Now get this, this Canadair camper van was inspected up the yahoo when I bought it this march and afterwards did 20 000 miles since then... I had changed ALL the filters, all liquids are regularly checked, no fuel pressure problem, no vacuum or air leaks, mechanical pump is fine, relay fuses as well, I can kick down no problem cause transmission is fine, even the catalytic converter has been changed !! Rev's perfectly when cold and so only after 15-30 minutes when hot does it start to miss fire and not with the pedal to the metal but with regular cruising power... H E L P !! This is my baby and home away from home when I do my craft shows.. As an artist, I aint rich you can imagine, so throwing away money on a mechanic to troubleshoot a problem is frustrating... Do you have any ideas ? Thanks a million for taking the time