I think I've found the problem: The left pump "surges". It operates fine most of the time, but when I run with it on only and it "surges" as indicated by a vibration on the left side of the bulkhead, the engine starves for fuel--quits--soon afterwards. When this happes, I switch on the right pump very quickly and the engine instantly comes back to life. I'll then shuft off the left pump and run on the right one for awhile. It will take a while, but the left pump will eventually operate normally and I can go back to running on it. Go figure?
I'm wondering if the problem is vapor lock caused by a restricted pickup, a pump problem, or somewhere else in the circuit.