What distibutor are you running? HEI? My 400/T400 did that twice and it was the module in the HEI both times. Ran fine until it got up to operating temp and then it wouldn't hold an idle unless you gave it a little gas with your foot. Ran fine at anything other than idle. What heads do you have? Those plugs are for later model heads aren't they?
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frittering and wasting the hours in an off hand way....
1969 GTO, 455ci, 230/236 Pontiac Dude's "Butcher Special" Comp hyd roller cam with Crower HIPPO solid roller lifters, Q-jet, Edelbrock P4B-QJ, Doug's headers, ported 6X-8 (97cc) heads, TKO600, 3.73 geared Eaton Tru-Trac 8.5", hydroboost, rear disc brakes......and my greatest mechanical feat....a new heater core. 
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