hot water circulating pump
I have heard of issues like this with Moen "pressure balance" valves, in circumstances where there is a pressure difference between the hot and cold supply (call Moen, they will tell you about it too). You said the valve was fairly new so it is likely a pressure-balancing unit, and this sound like a pressure-balance feature malfunction. Turn the pump OFF and test the system to see if the problem goes away.

If the test proves the pump is the cause then it may be oversized for the small single circuit on which it has been installed, maintaining an increased pressure on the hot-water side even with the water running, whereas optimal would be for the pressure to drop to normal system pressure when the valve is opened. As a matter of fact, removing any restriction to flow, as suggested already by others, might help (though, counterintuitively, for a totally different reason).

Grundfos re-circulators are powerful pumps, putting one on a small loop is not commonly done. More often a thermosiphon circulating loop is established, without a pump, and they work quite well. You could convert yours just by yanking out the pump and connecting the pipes up (make sure there is a check valve on the return line near the boiler, as mentioned by @d.george. 



Diagram shamelessly stolen from the internet