Cranks won't start

Have an issue 1 - 2 times a month. Truck will not start, will turn over but doesn't seem to catch. Can try and try no luck. Let it sit for 10 minutes and it will start and run great for several weeks and then the problem will occur again. Can happen day or night, hot or cold, just seems to be random. Not sure where to start. Possible fuel pump issue? Thanks.

Never had a problem starting.
It's zero degrees outside
I have a block heater.
Truck won't start, seems not to be getting fuel. It has been sitting for a couple weeks. When I first tried it it did start to run for a few (15) seconds. I wore out the battery trying to start it. I have a 50 amp charger on it now.
I tried to start it with some ether but I'm by myself and it's hard to do that.
Any suggestions?

Driving down the road at 45 mph and the engine stalls. Still rolling, I throw the truck in neutral and restart. The truck drives 1/4 to 1/2 a mile and stalls again. This time however, it does not restart. I figured it may be the fuel pump. What do the experts think?

Where can I find the fuel pump relay?

Thanks obxautomedic