I have a Satio 65 four stroke, runs lovely on the ground, no spluttering when it goes to full throttle, but once in the air after a couple of minutes goes dead stick.
It's been years since I've flown glo power but the principles are similar to a degree. It sounds to me that your fuel is foaming up because of vibration which will kill your engine. If your tank is not insulated from vibration with foam aroung it, that could be the problem. I've seen some guys put foam around the tank in a manner that is so tight that it is almost like solid wood. Your tank should have some give (flexibility)so as to absorb the worst of the vibrations. Also, unless you have a pump, you should have the tank as close as possible to the engine and the feed line pretty much in line with the carb. To put it another way, keep your tank centerline in line with the carb. Have you checked your fuel line for holes or cracks or poor fit from the tank to the engine?