A quick check would be to T in a fuel pressure gauge at one of the carb inlets and duplicate the climb. The pressure should remain high because the pump should be able to move more gas than the carbs could use. Maybe a marginal pump or partly clogged filter with the engine rpm down in overdrive, could not keep the supply up.
Are you running a hot spark plug? The long pull could cause the tip to over heat and cause pre-ignition.
If the timing is advanced for premium gas and is just a little too far, you might not notice anything in normal driving, but the long steady pull might make it show up.
Finally, again, if the advance is set for premium and you got some "not so premium"....
