Car Throttle Sticking problems of the 1997 Ford Ranger

During driving, when I shift from 2nd to 3rd (most of the time), the throttle sticks at around 2500-3000 rpm's. It does release itself but after I engage the clutch to the next gear, I can take it out of gear and leave in neutral and it will still be around 2500 rpm for about 3-8 seconds. I took the car in to the original dealer and they thought I was nuts. I took it to burts Ford in colorado and they first replace a fuel assembly part, the 2nd time they could not find the problem. They 3rd with burts, I took a mechanic out for a drive and shoed him the problem. They looked the car over while it was in shop for a week and said that they can willingly reproduce the problem but said it was the way the car was being shifted. No fix. I have been driving stick cars for over 26 years and have never had this problem. At one point, when shifting, the rpm's would accerlate. This is a problem, if I get to a stop light or sign and the car will not release the rpm's, then engine could continue to race and potentially blow up or even ruin the engine. This is obviously a safety issue but nobody seems to know how to fix it.