This is a long shot, but I had identical symptoms with a 140. It had sat unused for about 4 years and I was getting it roadworthy again. It tarted and ran fine, and would idle all day. But in 2 or 3 road miles, it would sag and then crap out.

The problem was a combination of rusty sludge in the fuel tank, plus an In-tank filter (which I learned later was no longer used). The road motion would put the sludge in suspension and it would clog the filter. After sitting a while, the stuff (mostly rust) would drop off and the filter was free enough to work again.

You have a filter (sock) on the in-tank pump. Maybe sludge in the tank too? Too bad you don't have a pressure gauge. That's what I used to track my problrm down, Had it rigged so I could see it as I drove, and saw it start to drop lower and lower, till it died.

The fix was to ditch the filter and put in a used tank from Arizona.
--
Bruce Young,
940-NA (current)
'80 GLE V8 (Now gone)
'83 Turbo 245
'73 142 (98K)
'71 144 (track modified--and still here)
New 144 from '67 to '78
Used '62 122 from '63 to '67