PCM was retarding the timing by 10-15 degrees.

Replaced everything and nothing worked.

Switched from 91 octane Shell gas to Mohawk 94 octane ethanol blend and...

Problem gone. Car will run fine on 92 octane as well with no knock.

 

I hate 91, but its all I can get around here !!! (toluene is my freind when I want to play)

 

Hey Grip, how much was your heads milled?? And was the block ZERO DECKED???

 

Try the old gas again. May have just been a bad tank. There is almost zero chance that octane has that much of an effect on idle.