Back to basics:

1. Are you totally sure that the pump is running even when the engine stops? Can you jumper the relay or otherwise make sure that it has continuous power and keeps running?

2. My other suspicion is control pressure. The engine gets enough fuel from the cold start valve to run and maybe stone cold has low enough control pressure to get fuel, but as it warms up the control pressure goes up to the point where the flap/lever can't move the plunger. Or maybe the plunger is sticking. With a stone cold engine and the electrical to the WUR disconnected, can you move the sensor plate with the slightest touch? Now connect the electrical. Does the plate get harder to move?

3. While the engine is running, can you keep it going by pushing the air sensor plate down slightly? Use a stick because it may backfire (don't ask me how I know).

4. Is the return line clear? Can you blow air through it and hear gurgling in the tank?

5. Is your tank venting working? Have you tried loosening the cap?

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe