1985 Eldorado that thinks it's a diesel?

Greetings,

1985 Eldorado, 25K miles
When highway driven at speed (40mph or better) for 15 minutes or more, call will detonate severly upon decelleration. Problem will go away if throttle is flored and kick down into passing gear. The detonation is severe and sounds like a deisel motor. The idle control motor will pulse the throttle to keep engine running. This problem only occurs upon decelleration to a to stop or merge into traffic after driving for at least 10 or 15 minutes at highway speeds. EGR replaced, fuel delivery test done, fuel filter changed, timing checked, car has new exhaust system and new catalytic convertor. Any help appreciated.

Thanks,
OB1-