I had a similar problem a few years ago on my 350 Chevy. Tried more things than I care to admit, to fix it, LOL. Turned out the polished aluminum (read imported crap) water pump had the impeller installed far away from the inside of the water pump casting. It would pump enough water to barely circulate, and fall flat on it face under any load. I installed a FlowCooler "high flow" brand pump and it was fixed. I suspect any "quality pump" would have fixed my problem.

I run a sealed hood and sides (with no louver's) and never even turn my electric fan on, unless it's stop and go traffic. Also pull a trailer with no heating issues. I would agree that nothing is worse than watching that needle climb on the temp gauge.

Good luck chasing it down.