So for the last two years now, my truck has had this ongoing issue that happens every once in a great while....it will be running fine, and then for no apparent reason, the idle will increase to about 2500rpm and stay there. When I hit the gas, it starts to sputter and spit, sometimes even stalling out. Lately, I can depend on it to happen at least once a day while driving it, and even though Ive taken it to 3 diff. mechanics...they say they cant find the problem unless they catch it in the act. Ive replaced the Idle Air Control Valve, and capped off the EGR as I was told this could be the problem....but it still does it! If anyone has any info on what this COULD be please let me know, its driving me insane.

If this helps... It is a 1988 Silverado, 350 5.7L (T.B.I. motor)