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One of the worst things that can happen during troubleshooting is to have a second problem develop during the process.
(I spent 3 years in the Army doing electrical troubleshooting)
The rough running may be related to the oil explosion getting oil in the wrong places.
Even an invisible coating of oil in conjunction with moisture may be causing the rough running.
At night, run the engine for several minutes to warm it up then spray a fine mist over the engine while it is running in the dark,
You may then see some sparks that are getting away.
You could wash the ignition wires, including the distributor cap and coil wire, clean the coil, especially the top.
(One of the Brickyard members puts his in the dishwasher!)
Then check to see if all the looms and parts that hold the spark plug wires apart and away from metal are present and in use - Most 740s that I bought used were missing nylon wiring clips and plastic wire separators.
As for oil pressure, I have even seen advice that if the oil pressure light is flickering, turn the idle up a bit.
Check for proper idle speed with a good tachometer - things like a dirty air filter, or wear and tear can reduce idle speed.
How old is the engine's general state of tune? Volvo computers can adjust for small problems until they get pretty bad and cause problems. If you do change some items, run the car for several days until the computer can adjust to the new items.
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