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Bob,
If your car is equipped with two relays that are identical looking and perched on the passenger's side strut tower try swithing them out with each other the next it won't start. If that is a solution you can then remove the covers of these relays and resolder the circuit board in them. Volvo's have a nasty habit of broken solder connections on their boards over time. Of course you could also spend a pocket full of money and just replace the relays with new ones if that is your choice. You should also locate your fuel pump relay in the relay tray beneath the ashtray and resolder it as well. If these two relays aren't where I describe then repost with a question about the location of the radio suppression relay on a 91 740.
If those relays aren't the cause.... when you are cranking the car in a no start situation can you see the tachometer needle "ticking"? There should be some slight movement as the engine is cranking over. If no movement is evident you might have a bad flywheel sensor. Or the ignition component that uses the impulse from the sensor might be bad. I don't know of any way to test the rpm sensor other than replacing it if the "ticking" isn't evident when cranking the engine. Good luck.
Randy
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