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I would also think you might have a main pump problem or circuit. When the tank is low, below half-full on fuel, the in-the-fuel-tank feed pump can starve the main pump.
Are any of the pumps very old? Any one of them could be sticking due to worn brushes.
The next time try thumping the pump bracket, as mentioned, with a hammer and use the technique of using a wire to jumped power over to the pumps from inside the fuse panel.
Look for a BB post or at Flametrap.com that covers which two terminals to put the jumper wire on. I know CIS cars have different fuse panel wiring. They are completely different from early and late LH systems. Like the left side of #4 to #9 or #12. Something like that will do it.
This will bypass the fuel pump relay and applies power directly to the pumps. If they do not start right up when power is applied then thump the bracket that holds the pump several times. If it starts at that moment, you found the location of the problem. It could be the pump but do not rule out a loose or badly corroded connection on or going to the pump terminals.
If the pump runs every time you jump the pump then you will have to consider a bad relay or circuit ground points. The computer turns on the fuel relay and the ignition system after it sees a crank signal from the distributor’s hall sensor. Checking for spark first always shifts the direction of the shotgun. I mention this just in case you started your hunting trip with out a complete terrain map.
I will try to think out some logic here but like a doctor, no cure when practicing.
Because of the described regularities with long intervals in between it running correctly, to me, it seems a whole lot like a tired main pump.
It did not quit going down the road. It happens more times that way than not.
Electric motors pull more current when starting.
Add in worn contact surfaces that will limit the kick needed to get it turning.
Cranking the engine lowers the current through out the system to begin with so the battery system itself can be a variable.
As the other poster said, a bad check valve on the outlet of the pump can happen because the little spring inside can wear thin on one side and thus becomes weaker so it is not quite broke.
My experiences taught me all three of these scenarios over thirty years of owning electric fuel pumps.
Just like the old clocks, they finally whine down.
Phil
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