My guess would be a restriction in your hot water system somewhere. Set your shower to all hot and turn on the pump. Pump into a bucket and see how much water you get in some time, say 30 seconds. Then do the same with all cold. If they are not close there is a restriction somewhere. When the pump goes off, pressure drops and the flow in the restricted line drops faster than the unrestricted.
The blend valve is a good suspect. Before i bought a new one i would take it apart and see if the hot orifice is blocked.
Like i said, just a guess.
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