If you are still running the stock gas cap, it could be plugged up, and not letting air get into the tank, as the fuel is used, causing a "vapor lock". Another possibility is, if you have the stock breather tube on the gas cap, and it is run into your steering stem, your steering stem could be full of water, not allowing the tank to breathe. I ran into that last problem, on a Polaris Scrambler, and it took several hours to find the problem. Check those things out, and let us know.