Valve cover bolts are Your main/pump relay may be failing intermittently.
So, you sis warrantee one through Walbro. Did they have any comments? The Reason I ask is that they test them before doing anything. They are a very reliable pump.
I would, make sure your wiring/connections are good. I had one and use a 35 amp relay off the power from stock relay and ran like 12 or 14 gauge all the way back. They can be a big amp draw and melt those litle 16 gauge wires somewhere.
Try tapping the pump with a hammer. I did se 1 Walbro that the armature cocked in an ceased up. SmaCKED IT LOOSE AND IT RAN. THEY ARE LOUD.
CHECK VOLTAGE ST WIRES THAT CONNECT TO PUMP TO MAKE SURE YOUR GETTING 12 VOLTS. Ery light on the torqu. I think some are in inch/pounds. You don't want to squash the gasket flat.