fuel system problem

ok I have a 91 ford f-150 and it has dual tanks I went to start it up
one day and I didn't hear the fuel pump kick on when I turned the key,
and engine would rotate but it would not start. 2 days later I tried it
again and the fuel pump kicked on and the truck ran fine and then when
I turned the truck off the problem came back again. I mix and matched
the relays and I made sure the inertia switch wasn't popped, but still
I can't get the fuel pump to kick on. I don't want to replace the pumps
if I don't have to, because when the pump's kick on the truck runs
fine, also I have dual tanks and neither of the pumps work and then
magically it will just kick on and run fine. Does anyone have any idea?
should I replace the inertia switch and get all new relays? any ideas
are welcomed.