Tobin,It would help if we knew what you did when the engine was "pulled and sealed".What parts were changed?
Could there be a exhaust restriction when it has this power loss?Is there any change in the exhaust sound?Loose things in the muffler can cause intermittant blockage but usually require a shutdown to return to normal.
Does the temp guage work and does it indicate normal temp at this time?
It sounds like you could be experiencing piston seizure maybe from overheating.
Will it idle when it has this power loss?or do you have to keep the throttle open to keep it running.
If the brakes were dragging it would be detectable by the brake(s) heating up and when you release the clutch while on the road the engine would run normally but the vehicle would drag quickly to a stop.
Time for a war story;
In the late 60's with the natl guard in convoy to Ft Stewart Ga with the usual equipment of the day which was left over from the Korean war days.We had several GMC deuce and a halfs and they were sure tough old trucks with their 302 hooked to a hydra-matic box.Loaded and hooked to a loaded trailer they would put the speedo hand 2 spaces past the last numbered space which was 60 if you were on a good downhill run--and they managed to use less fuel and keep up better with traffic than the Dodge power wagon type 3/4 ton truck or ambulance.Had one throw a fit where it would start losing power and get down to a crawl forcing the driver to stop-he would shutdown and check his engine and when restarted it was back to normal for a while when it would do the same trick.My truck was behind him so we pulled off too and after the second time I got in to drive it looking for a repeat performance and I didn't have long to wait.When i pulled off the road I kept it running but not easily and it was leaking exhaust from several joints between muffler and engine with a loud hissing sound.Well the problem was obviously a plugged muffler so we dropped a section of pipe loose and they sure were obvious for the rest of the trip but I think they changed driver and assistant a few times to get a rest from the noise.A muffler replacement cured it.We loved to go through Atlanta in convoy as fast as we could run and cut the switch off-turning it back on at a appropriate seeming time and watch the expressions on people's faces in nearby cars.Some of these things would make a hellacious bang and occasionally the muffler would split at the seams or be swollen totally round instead of oval..The M151 jeeps were great at this trick

Bob Taylor