What the heck is a "fuel relay switch"? And how did you eliminate any electronic fault? Just because there aren't any codes stored or there's no "check engine" light, doesn't mean the problem isn't electrical in nature.
Sounds like Relay 109 problems, but that isn't what that relay is called! If this relay conks, you will NOT get trouble codes stored, it's exactly as if you switch off the key, and then it will intermittently allow the engine to start and run afterward as if nothing had happened, until the next time the relay cuts out.
In general, mechanical problems with the fuel supply system will not cause the engine to intermittently cut out without other symptoms present. Fuel starvation will cause gradual loss of power. Leaks of air into the fuel system are easily visible as gross amounts of air bubbles in the clear plastic fuel lines. Major mechanical failure will stop the car dead and that's the end of the story, it won't start up again.
I'm pretty sure you have an electrical problem, but you need to keep looking!
Brian P.
'96 Passat TDI mit UPsolute
