Hi Tom,
I don't really have any good suggestions -- sounds like you've done all the reasonable things -- but here are a couple out there thoughts -- the kind of things to try when the reasonable stuff does not work
Just as a diagnostic idea, one thought would be to slow down the drain back and see if the air lock is due to the system going through the drain back process to rapidly and somehow entraining some air that gets into your pump.
Maybe just temporarily add a restriction to the return where it ends in the tank. This would slow down the drain back by not letting air up the return pipe as fast, which will make the system drain back more slowly.
If this solves the problem, then I guess you would have to figure out a way to slow down the drain back while still making sure its complete.
Another thought would be that there is an air leak in the system somewhere that is pulling in some outside air during the drain back. I know this sounds unlikely, but remember how strangely the air leak in Thom's system made it behave.
Gary