It is as said almost defo a sticky relay, turn on the ignition wait for the normal amount of time for the prime to stop, then with ignition left on tap the relay for the fuel pump, and see if it stops, if it does just change relay, if not it could still be being fed by the ecu, or it could be totally jammed, put a test light on ecu input to relay and see if the test light lights only for the prime time, this will soon determine whether it is a relay or ecu causing it.
tabetha