I wanna make a WAG too...
Side play in the distributor shaft...
but I also like the wag of vac. advance sticking, due to; too long of a screw used to hold the vac. diaphragm on to the distributor.
but wait,,,
let me ask you, does this air filter housing have the 'air horn' on the air filter housing intake, (where air first comes in), or does this airfilter housing have an 'up-high' clean air intake.? (?missing air-horn getting filter element too wet?).
And if this truck is ever used outside, can you correlate a connection to rain the day before the miss?