Similar problem this past year on my '90 760 GLT.

Made certain that battery was strong, grounds were clean and battery cables were not corroding. With this part of the starting circuit ok'd, I went to a second part of the starting circuit, the ignition switch circuit. Here, I disconnected the "thin wire" connected to the solenoid's terminal, T50. I measured for nominal battery voltage from that wire's end to ground when the ignition switch was switched to START. If I got nominal battery voltage, that part of the starting circuit was ok'd. All that remained was the starter motor/solenoid. By elimination of the previous two, my culprit was that starter motor/solenoid.

Yes, my starter motor/solenoid was brand new. With the help of a remote IR temperature sensor purchased from Harbor freight, I proved that the car would start anywhere below a threshold temperature of 108-110 deg. F. It would not start above that temperature.

When you can't start the car after the engine compartment has been heated, direct a thin spray of water from a garden hose at the starter motor/solenoid to cool it way down. Then, attempt to start the car. If it starts, it's that "new" starter motor/solenoid.

For the fun of Volvos,
Mike
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'92 245DL, '90 760 GLT