Help Diagnose Starter Problem

[adis]

I've got a starter problem on my sbc400. it's a rebuilt starter that I just bought. when i turn the key the solenoid engages the pinion, the starter motor turns and the fllexplate (FP) turns about one rotation (sometimes a bit more, sometimes a bit less) then abruptly the the FP stops rotating, as does the pinion, but the starter motor keeps spinning. The pinion continues to be engaged in the FP until I turn the key, at which time it snaps back.

Video here: https://youtu.be/A1fHLSCI_hc

If I keep trying, after about 25 bumps, I can get the engine to finally turn, and it idles fine. It's a new HEI dizzy, I set initial at 12*, mechanical at 34*@2800, and vacuum at 46*@3000. I took apart the starter, and the bendix/pinion seems to be OK - one-way clutch works, the threads on the shaft appear fine. teeth on the pinion are neat. I volt checked the batter terminals, ~11.5V, ~10V under load. I checked the battery terminal on the starter (the big one) ~10V under load. It's a new flexplate, Scat brand supposed to be slightly thicker steel, stiffer than stock.

I;m inclined to think that it's "starter drag" that I've read about on other sites. Doesn't seem to be electrical - seems to be mechanical. After about one rotation the engine just won't let the starter turn. It doesn't kick back, but it did when I first installed the dizzy, and hadn't set the timing yet.

Any thoughts, opinions are appreciated - this is driving me crazy. Thanks - mark