Motor stops after 1/2 mile

[Richard Cranium]

2008 Yamaha Vino 125, 16k miles

It starts easily but the engine cuts out about 1/2 mile down the road and it takes a couple tries to get it started again. It then only travels about 500 feet before cutting out. The distance continues getting shorter between starts as the engine heats up. No smoke, no apparent leaks, and no sputtering. I have been trying to track this problem down for over two months. I have replaced the carb, coil, CDI, all vacuum hoses, fuel cock assembly, and fuel filter. The fuel line appears in excellent condition. I am at my wit's end. Can someone please give me an idea where to turn next?