I had a problem like this on a cortina, using a mt75 dog box but was running a quarter master, and I found it was the release bearing getting stuck on the input shaft slider as it hit max pressure, we had a machine shop make an extended outer shaft(sleeve),

This was a 2.1 pinto running a slimmer lightened flywheel, 7.25 quarter master clutch, and 5 spd dog box, so unsure on your setup but it can't be to far off?