If your 900 is about a '89/'90 model then it is probably your 'crank angle sensor' AKA 'hall effect sensor'. It is a small sensor mounted on the crank pulley (bottom of the engine, next to the firewall. Hard to get to!) that sends a pulse to the computer to let it know how many revs your motor is doing. When these get old (as they now are) they decide not to work when they get hot (strong union!) One way to confirm this as the fault is to spray some freeze spray on it when the car stops and see if it restarts immediately. I've changed a few of these on my wifes car so if you have any more questions let me know.
Kalon
March 2008
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