misfire under load.
I was browsing as a new member and curious as to what subjects are discussed and found your forum. The only answer I saw as correct for your particular symptoms was a bad cam which can happen very quickly on a new motor if certain things come into play. If you checked to see if a rocker arm seemed to not be moving a proper lift that would indicate a worn cam lobe. These things happen and I'd be intereted in knowing if you did solve your problem. Phil D.