OK. Let's assume since the radio is new, and un-moddified that the problem is not the radio. Let's assume that you know how to check SWR. I'd start looking for intermittent issues with the antenna system. From the rig to the ball at the top of the antenna. Make sure everything is well bonded (grounded) and double check the feedline. Can you beg, borrow or steal a Dummy load from one of your Ham neighbors?