Problems starting after driving on 1994 Volvo 960

I have a 1994 Volvo 960. I can go out and start it right up, but after I drive it, it won't start again until an hour or two later. It is like the engine has to be completely cool for it to start. What could this be?

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1 answer
RPM sensor, fuel pump relay, coolant temperature sensor, fuel pump. Any of those can do it, check the diagnostic trouble codes and post them for more info.