You can check everything but piston skirt clearance and piston ring gap by pulling the oil pan. Drop the pan, and check rod and main bearing clearances with Plastigauge. The ideal clearance is somewhere around .001"-.002" for rods, and possibly slightly closer for mains. You might also try pulling the spark plugs quickly after a warmup, and checking the cranking speed with the plugs removed. If even a single compression test load slows it down, I suspect an internal problem in the starter after warm-up. Ditto on the suggestion to swap starters and see what happens.
Jerry

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