I can't believe I'm the one responding to my own thread, and I can't tell you how much I hate to state this but ....... I just found online in a Delta manual, a comment about NOT using PEX between the valve and the spout. Further searching revealed that 1/2 copper inside is larger than 1/2 PEX inside, and there's a reduction in flow which will cause water to rise! Does this sound like the culprit for my situation? I dearly would have loved knowing that when I was first plumbing the tub/shower. Unfortunately I was given the parts over a series of visits and never had the manual. I would still greatly appreciate any advice. If this is the problem, I have access to the back but haven't a clue how I would get behind the 2x4 cross piece and access the short PEX line!!!