Originally Posted by BPettit

Be sure to check your pipe as someone has already suggested. If the pipe has any restriction , the restriction will be at the smallest point, most likely at the exhaust press port. When your tank has more fuel in at, like at the first of the run, it may not take as much press to push fuel to the engine. 4 minute is you have less fuel, more air, fuel may require more pressure. Exhaust port on the pipe may not be completely closed, but may be clogged enough to reduce your pressure and cause a problem. Just a theory, but I've cleaned a lot of pipes and you would be surprised what comes out!