wewille

My problem is while showering the water will turn cold after a little while. I had a heating company rep. work on it and he installed a check valve in the outlet line going up to the heat exchanger unit in the attic, This did no solve the problem. I replaced the hot water tempering valve spring/knob etc with service parts(Watts Series 70A) with no help. I also replaced the Hot Water tank with one of the same specs, no help. This unit was installed when the house was built in 1989. I inherited the problem when I bought the house a couple a years ago. Does anyone have an answer for this?