Diamondback and David,
Thanks for taking the time to reply. The sled detent was adjusted a little more loosely than I think it should have been, so I tightened it a bit. Barrel was clean. Checked everything else for tightness, put the action back in the stock, and started shooting again. Action screws torqued at 10 in-lbs. Scope mount rail at 25 in-lbs, ring halves at 20 in-lbs.
Had the same POI issue with the Vortex scope, so mounted a Nikko Stirling Target Master and seems to have resolved the problem.
The Vortex Diamondback lasted about 700 pellets. I'm going to put it on an FWB 150 and give it a try before I send it back for repair.