Just whizzing through these posts I didn't see any mention of the possibility that air is getting drawn into the tank side of the fuel feed which might only be a problem at tick over RPM.
I say this as my first van - an 1,100 cc side valve Fordson wooden bodied "shooting Brake" would stop (I though at the time) due to vaporisation at the fuel pump which I could cure by giving the fuel tank the kiss of life and myself a clown mouth. I never really got to the bottom of it though.
As an amusing aside there was no MOT in those heady days and the rod brakes needed a knees to chin pedal travel and you could hear the multitude of slack rod pivot bearings and mechanical equalisers taking up to initiate braking just before the pedal hit the floor. Plus I had to wiggle the steering wheel after a tight bend else I wouldn't be ale to open the rear doors as the body framework had wood worm.
I nearly killed the driving test chap as a giant drum of horse hoof brown glue (from my dads' business) rolled into him on the emergency stop and shot the temporary passenger seat - an armchair with the legs cut off, complete with tester under the dashboard.
I guess he passed me in case he should get me on a re-test.

Well-timed silence hath more eloquence than speech.
"A quiet shy boy who took little part in games or sport"
88 High top 2.1 WBX