Could it be a broken valve spring, The inner damper could be allowing the valve to close at low RPM but bring it up and it can no longer be controlled, I've seen it on a few chevy's like that
70 cuda 440 6bbl Auto with a dana 60 68 road runner QQ1 383 auto 1970 6 barrel 4 speed Superbird FY1 number matching 15 Challenger 6.4 SRT pearl White 6 Speed 72 Cuda 440 5 speed 71 Road Runner 440 4 speed
