Hi fifthwheel,
My information about blowing fuses is not hypothesis / supposition.
Fact: A fuse is a protective component in a power supply.
In this case it is a 12volt supply and the 80A fuse is in the +ve line of the circuit, it protects other components in the circuit from damage due to high current flow.
A calculated level of current must flow when the components in that circuit are connected to the -ve (ground) side of the supply.
An abnormally high current will result due to a full or partial short circuit to ground. At this point, the fuse will become an open circuit.
The relay is controlled by the ECU which is not in the 80amp circuit. The time on to off is correct. Forget it!
END.
This is hypothesis.
From your information, you have excess current flowing intermittently in the 80 Amp circuit.
The fault lies in the glow plug circuitry beyond the hot side of the fuse.
When the buzz bar shorted, connecting wires could have also heated and could have destroyed some insulation. Movement can cause the fault to come and go.
I refrain from calling this a short circuit because the fuse is not blowing immediately, just overheating. I suspect a damaged insulator is 'carbon tracking'.
So trace back from the glow plugs to the relay and look for any carbon deposits, burns, soot etc.
If by chance a glow plug is at fault, you can isolate them by disconnecting the bar for a while. However, I favour the former due to the original short.