My '92 240 is hard to start sometimes in warm weather. The problem is especially pronounced if I have been running a series of short errands where the car is stopped and started again several times in one hour. I have replaced the RPM sensor and the fuel pump relay, but it still does this. My guess is something electronic under the hood is failing when a lot of heat builds up under the hood, but what? The problem does not show up in cooler weather. Anyone had a similar problem?

Lawrence likely has the nail hit right on the head here. The temp sensor can definitely cause this problem. While you're checking the sensor (not the sender, they're two different things) make sure the harness plugging into it has good wiring connections, and there's no corrosion in there. Good luck finding the culprit.
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Isaac Babcock - '83 245DL 'Borkie' and '83 244 turbo project

check the temp sensor.