checking the fast idle valve is definetley good suggestion. where i work we get cars coming in all the time in limp mode (reving up and down between 1 and 2 k). just take one screw off the top and loosen the other and swing the cover out of the way. then get a flat head and put it in the slot and turn it clock wise (do not over tighten it) that platic peice inside just loosens up overtime