Tom,
Sounds like tolerances are a little too tight for either the pistons or the bearings, or perhaps a combination of both; thus when things are at operating temp, stuff expands enough to make it tighten-up.
Have you tried pulling the plugs and turning it over when she "sticks"?
The old Chevy shop manuals had a method of adjusting the bearing clearances utilizing the starter and an ammeter; I'd suggest cheking your shop manual and reading that particular section and checking out that end of things (no puns intended).
I 've also seen in much older shop books, re-built engines being "run-in" by belt in the shop, to "burnish" everything to a running fit; but that was definitely in the days of poured bearings, etc. Shouldn't need to do that with today's insert bearings.
I beleive you said that the new crank bearings were "align bored" or reamed?
That was necessary with the OEM shell-type main bearings.
If the bearing clearances are okay, you might want to pull the head and check the cylinder walls for evidence of scuffing, which would be caused by piston binding or inadequate lubrication.
Good luck!
