i had a small block in my bracket car that did the same thing (318 or 360 i can't remember). i checked alot of things but never did find anything to blame it on. 1 old timer told me he had a ford motor do the same thing and after 3 rebuilds he discovered a crack in the lifter galley which would open some and bleed off presure with engine heat. of course there is no way to fix that so i would run the motor as is and start gathering parts for a backup motor if this 1 goes belly-up 3ddart