Thank you everyone for the suggestions. It actually did end up being loose drive belts. They weren't squealing, and didn't show any signs of glazing, but, having eliminated a number of other things, I tightened them, and presto! No more slow down during heavier cuts.
Special thanks to Mark Jacobs (again) for giving me several suggestions, and answering my questions.
Special thanks to Mark Jacobs (again) for giving me several suggestions, and answering my questions.