Years ago I had a customer bring me his 1968 383 C.I. Charger that he had put a set of after market cast aluminum valve covers on with the stock type PVC and twist on oil filler cap with no vent. It would start and idle fine for a short while(1 1/2 to 2 1/2 minutes) and then slow down by several hundred RPM, the motor was sealed up tight enough that the PCV was making a vacuum in the motor after it suck the pressure out of the motor I put a breather cap on it and he was happy after that A vacuum pump will do the same thing on a leak free motor The pumping action of the pistons and rods going up and down and ring blow by will cause crankcase pressure, especially at high RPM
That's funny Cab , 20 or so years ago I had similar issues on a customers stock 308ci Holden , after 20 minutes the motor would stall . Took a little while to work that one out , vented oil filler cap solved it .
Tex
New best ET 10.259@129.65 . New best MPH 130.32 Finally fitted a solid cam, stepped it up a bit more 3690lbs through the mufflers New World block 3780lbs 10.278@130.80 . Wowser 10.253@130.24 footbraking from 1500rpm Power by Tex's Automotive