I tried to pay closer attention to all of these suggestions since I posted this trhead, and came ot the conclusion that there are two things going on. One was my resting my elbow too heavily on the body of the guitar - somewhat tough to avoid since it's so large. I have to just adjust that posture myself though. The other was the headstock wanting to drop towards the floor the moment I release pressure with my fretting hand. I fixed that by replacing my cheapo nylon strap with a wider leather one.

Thanks for all the advice and suggestions!

=k