Keep it simple....I'm with the fuel starvation mob. Check the filter on the top of the carb. I had similar symptoms when using a bike after a long lay-off. The filter was full of cack. It let enough petrol through for low speeds, but not enough for sustained higher speed. Stop, wait a few seconds, and it would start again fine. Up the speed, engine splutters and dies. Not a seizure, just a lack of fuel. Clean filter and problem solved - in my case at least...