[Bray Hill]

I believe the problem is your egr valve. EGR is added to the intake charge to reduce NOx emmisions. In late model t-birds they use the EGR to improve fuel economy by adding a lot of spark advance when the EGR is enabled. A rough rule of thumb is that for evey single percent increase in EGR, the ignition is advanced 2 degrees to offset the slower burn due to the dilution effect of EGR on the intake charge. Therefor if your engine is running 10% EGR the timing is advanced by 20 degrees, and if the EGR is not present the 20 degrees ignition timing advance over non-EGR mode will most likely result in predetonation. This could be caused by a failed EGR valve, restricted EGR feed port, loss of vacuum signal, or so on. EGR is only fed into the intake at high rpm or throttle levels wich explanes your full throttle predetonation problems. Also high octane fuel burns slower and works better at higher spark advance levels and would help to reduce predetonation or spark knock, whatever you want to call it.

Drive it like you stole it.