Vespa simply dies twice

I have a Vespa P150X which I basically used as my daily transport. I have several bikes in my collection and I have owned/restored more than 35 bikes in my life. I therefore do know my way around bike engines which makes this more baffling. This vespa was completely overhauled 7000 km ago and has been running like clockwork every day. One morning I took off from home and a about 400 meters from the house it simply died. Ignition (electronic) working perfectly with good spark. Good fuel supply. Removed carb and cleaned it properly, replaced flywheel oil seal--no luck. I thought that the clutch oil seal might have gone. Opened engine and found no fault. I replaced all seals and bearings, new piston and rings, honed the sleeve which is still as good as new, fitted a new clutch and brand new Del Orto carb , a new exhaust and sparkplug. Bike started second kick and ran beautifully again only to do the same thing 500 km later--just died. Took the tank off. Cleaned the whole fuel line (which was not dirty at all) and checked everything again. Good spark, fuel supply and compression but stone dead.

This is truly baffling. Any suggestions

Best wishes

Louis Gouws