It could be something blocking the fuel flow path through the carb. If your sure your getting a consistent fuel delivery to the carb then check the carb. Pull the needles out and flush through the paths with some denatured alcohol, or blow them out with compressed air. Visually inspect all of the needle fixtures, the needles and the o-rings. Reassemble and see if it clears up the problem. I don't think this is the most likely cause of your problem, as carb blockages are usually pretty consistent, and screw you up all the time rather than it working OK and then going nuts all of the sudden after a few minutes of running, but it's so easy to pull the needles and thoroughly inspect the carb that there is no harm in doing so and it will at least eliminate one possible cause of your problems.