2002 Ford Ranger Edge

automatic shift truck suddenly revs, the rpm needle goes way up and I have nearly gone off the road and into a ditch and once during breaking it revved and i nearly rear ended another vehicle. There is no warning that this is going to happen. We have learned that if we immediately shift into neutral or tap the brakes 2 x it stops and the rpm returns to normal. We have had it serviced several times but it never does this when the mechanic is testing it. I am ready to trade it in as we are at a loss at towhat causes this and are afraid it will cause an accident. it only has 40k and otherwise is sound