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I have an 86 740 (along with some other Volvos) and I think you're going to find it's either the fuel pump relay (they act up when hot), which you can try taking of the cover and resoldering. I think its behind the ash tray and fuses, second row on left (you can check the 740, 940 FAQ to be certain). A new can be had for around $28.00 to $35.00.
If this does not cure, then the much more expensive distributor rotor hall effect switch needs to be checked. Sometimes its just the wire coming from the distributor hall effect that get brittle and cause a short. Its a flat three wire connection running from the distributor cap. Also the o'rings seals get hard over time and oil may get into the hall effect assembly.
You can check the hall effect operation at the connector (at the firewall) with an ohm meter or continuity checker. The the FAQ for more information on this to get the right wire color combination to check.
If you do not have a meter or continuity checker, you can remove the distributor/hall effect assembly (not the entire distributor), using a rachetting open end is best with the tight fit, and check the hall switch operation. However, keep in mind that you may want to first get new o'ring seals in case they are brittle and might crack (there is small one for the shaft and a large one for the switch assembly). Removing all the plug wires and coil wire and pull out the unit. With the key in the aux position and rotating the metal vane assembly, you'll hear the injector solenoids going off and on if the hall effect is working properly.
Gary
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