Unless I misunderstood your post, you replaced your fuel pump/sending unit with a used one with 90K miles on it and now your fuel pump runs intermittently? I'd suspect your in tank fuel pump is or is going bad again. If possible I'd check/probe the wires when the problem is occurring to eliminate a wiring or connection problem.

 

Yes, I replaced the whole assembly, I was told it was my fuel pump that had want bad the 1st. Time, I was told it was because I seldom drove it and I always left it pretty low on fuel. I know this probably doesn't matter much, but I bought it new and had never put anything but high octane 93 or higher in it. So I did replace it with one that had higher mileage, out of a wrecked coupe, it had higher mileage but was advised that was a good thing in that scenario meaning it had been driven and used regularly. So now when I start it, sometimes it starts with no problem, others times it sits and cranks but doesn't start. The times it starts, when I turn the key to acc but havent turned it completely(key start, not push button)I can hear the hum from the assembly unit under the back seat. It starts everytime. When I do the same thing and I hear no hum, I know it's not gonna start, so I'm assuming that's the fuel pump priming in a sense the lines from the tank to have it start up, or possibly the fuel regulator since I was also told it is in with the fuel pump assembly. The hum is around 3 seconds long.
I did change out the assemblies myself. There is the chance I didn't connect the wiring as securely as it was factory set. I'll try that with the wiring and see if that's what it could be. Like you said, it's intermittent and doesn't do it every time, but when it does I have no idea how to correct or fix it at that point and usually 15 20 minutes later after inserting the key 100 times turning it to acc and then back to initial spot when I place it in the ignition, and pulling it back out.
The other major problem that has caused is that it also does that while driving it. It just bogs way down and absolutely no reaction from the gas petal, pump it, do whatever, but it's got no response and I just coast it to where I can get off the road.
Thank You, I appreciate you taking time to give your thoughts and reply back to me. I'll look at the wiring and go from there, thanks again.

Unless I misunderstood your post, you replaced your fuel pump/sending unit with a used one with 90K miles on it and now your fuel pump runs intermittently? I'd suspect your in tank fuel pump is or is going bad again. If possible I'd check/probe the wires when the problem is occurring to eliminate a wiring or connection problem.

 

Have swapped another relay in place of the fuel pump relay? Not much else other than wiring.