Typically when something purely electical fails, it fails completely.

Electromechanical devices such as relays and the fuel pump in this case can act like this when they fail. Since you have already replaced the relay, it most likely means that the pump is bad or something is intermittently not allowing power/ground to either the relay or pump, (like the ignition switch for example) It could still be the relay, but it is unlikely.

This is how I would troubleshoot it. The next time you are able to duplicate the problem, check to see if you are getting power at the pump w/a volt meter. If not, you know the problem is "upstream" (electrically speaking) from the pump. ie. at the output of the relay. If you are getting power at the pump, then check to make sure the pump is getting a good ground as well (use an ohm meter for this and check for 0 ohms between the ground wire at the pump and a good solid metal portion of the chassis). If it has a good ground, replace the pump. You may also be able to tap lightly on the pump with a screwdriver handle (kind of like freeing a stuck float in a carburetor) and get it pumping. If that works it tells you that it is a bad pump as well.

Now if you don't have power (voltage, not really power, but anyway..) at the pump, check it at the output of the relay if power isn't there, then I would starting looking at the control circuit for the relay. If there is power, then you either have a high resistance short in the line to the pump from the relay or a loose connection somewhere. (In other words, a wiring problem).

Finally (and this one would really be a stretch) check to see if there is something else magnetic that has recently been placed by the relay or perhaps a wire that is carrying an unusually high amount of current through it (this creates an electromagnetic field around the wire) which can cause interferance in the relay's operation. Again, I would find this EXTREMELY, HIGHLY, INCREDIBLY unlikely, (Did I stress that enough?) but it is possible so I will throw it out there.

I hope you get it running,
Firechicken