Is it supposed to breathe through fuel filler cap ? If so ... maybe this is blocked ? Is there an inrush of air if you remove the cap when problems arise ?
Why not take sick bolt to local fastener supplier and get something the same size. Doesn't have to be same thread. Or phone Mick at Redline Components for reasonable postal charges.
It would be scuttle off, not dash out. Maybe you can see a screw near or piercing the loom with a phone camera .... take a few pics with flash on. Guess you know from AA man intervention which part of loom to narrow it down to. Good luck.