Engine Surges HELP

We have been having similar issues with our 1994 tbird. We also have an issue when cruising (not cruise control but consistant speed) On the highway. It will rev up to 3000+ rpms and stays like that for several seconds or when the gas pedal is released. We have taken it to our mechanic and The ford service dept. and they cannot duplicate it or find anything wrong to cause it. Any help would be much appreciated. It's getting worse by the week.

Thanks.

DKM