I helped a friend trouble shoot the same type of problem on his 1.3 scale Hawk's UAT. Same brand after we switched from a large CAT. It turned out to be that the fuel tank's vent lines were offering to much of a restriction. It was plumbed with 6mm line and large fittings. We were suggested to either go to a larger vent line or just double up with two complete vent lines. We added a second vent line and this solved his problem. He was running a large motor, but I can't recall the mane of the motor at this time.