I think I got it! It was your comment about the graphics chip possibly overheating. iStat menu was showing it at over 70 degree (C) when the program fps was low. So I got to thinking: "Why is it overheating lately, when for months I could run for an hour without going below 100 fps"? Then I realized I haven't cleaned the fans on my computer for a long time. I cleaned them out (lots of dust), and so far things are looking good again. The GPU is *****g at about 65 degrees, and that seems to be enough to make a difference.

Thanks for the help.

David