could be an exhaust valve sticking. i have found that an old engine
will build up crud on the valve stems at heavy load they tend to open just a little further than at idle and stick for a fraction
of a second. i would pull the cover rotate the exhaust valves
open one at a time and tap them (not frail )with a hammer they
should have a live hollow sound and the hammer will bounce.
if they sound dead they are sticking some good pb blaster and a little taping will free them up if they aint bent