My guess is that it is not the mixing valve. Even if stuck, it would be consistently and continuously the same. Maybe too hot, maybe only warm, but consistent.
My other guess would have been the stir pump, but you have a new one. But, even a new one could be defective. I bought three in 12 years!
My last guess is that it is not staying lit. If you are running hot water, the burner should be running constantly. Is it running?
The HydroHot is a wonderful system when working. But, there are so many interdependent parts, that only a very few technicians could figure it out if it was malfunctioning. It is too complex for normal people to understand and there are many models.
The last time (of several) that ours broke, the technician looked at me and said, "A complete trouble shooting will take me a couple hours. Are you willing to pay me for that?" I did. Among other things, the ratio of coolant to water was way off the target after years of use. The burner tube was cracked. And the stir pump worked, but only at about 50%. When he was done, the shower returned to being a wonderful experience.
Get a good technician!