OK - just wanted to close up this thread for anyone reading it in the future. i'm 99% sure that the improperly seated trans connector (on the passenger side) was the problem all along. after getting it cleaned out and seated properly last week, i had it on several long drives this weekend without any hint of the power-loss issue coming back and the trans still shifting better than it ever has since I've owned it.

with this identified as the problem, and looking back on the symptoms, i would say it seems one or more of the connectors controlling the shift and TC lock solenoids were just not engaging at all, or sometimes at random. i remember at times wondering if i really had 3.55 gears in the rear like the door sticker said, since i'd be cruising at 50-60mph, but the RPMs were up over 2k. i guess the TC was just not locking at those times. then, the bogging down that i'd get at low speeds sometimes was probably the trans being in too high a gear or locking up the TC when it should not have. and all the while, the PCM had no idea what was going, so no check engine and no OD light flashing.

anyway, glad i got it fixed - now onto the fun projects with the truck!