Forgive me if this is patronizing, but I don't have a sense of how comfortable you are with diagnostics so bear with me if I am over explaining. The combination of cutoffs while driving, and intermittent refusal to fire up sounds like a wire rub or similar. I went through 2 healthy pumps and numerous in-line filters before figuring this out. At very least, it is a satisfying cause to rule out. Carb problems don't typcially mask themselves with good performance in-between hiccups. Electrical shorts, on the other hand, are evil. With a shorted fuel pump, during startup, the vacuum from the engine can pull enough from the lines to get it started, and even driving, but it will struggle and cough as the wires jiggle and the pump gets a few rotations in here and there. When driving, the same principle applies. Hit a bump and the pump quits, but there is enough fuel in the lines and enough vacuum in the system to struggle along - maybe even enough to start it and hobble off of the freeway. You check everything, and there seems to be fuel in the lines but it refuses to start. Then, suddenly, it will just turn over and run like nothing ever happened. Frustrating to be sure.
First thing I would do is check the quality of the fuse for that circuit (relative to the other fuses around it). Is it darker or different looking? If the fuses look good, hook a 12v light bulb to the pump contacts and jiggle some wires around to see if there is a rub somewhere along the harness? If the bulb blinks while you're doing that, there is the smoking gun. The bulb flash will be a lot more obvious than the struggle of the pump's 'tick', especially with a bunch of engine nosie present. See if the pump can wiggle around on its mount at all, and whether that changes anything. Ultimately, you will need to jiggle everything between the pump and the fuel system fuse(from the pump through the the chassis harness to the engine bay, and along the engine bay harness to the fuse box. If that doesn't produce any weirdness, then like the others said, begin troubleshooting the carb.
_________________ 75 Mikado G180/4sp Desert Tortoise
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