I have a newer home about year old, ever since we moved in we have had this problem. its a two bath home single level with the master at the far end of the garage were the hot water tank is in the garage. We have a Grundfos UP10-16A pump just for the master bathroom. The plumber installed the pump at the hot water tank return the water into the bottom of the tank were the drain was I looked at the installation direction on line at Grundfos and this one way the show it to be done. The problem is the two lav's sinks in the master work great instant hot and stay hot but the shower which has a Moen shower mixer valve, comes on gets hot fairly quickly but then gets cold and then gets hot again so your adjusting temperature the whole time you are trying to shower. I have rebuilt the shower valve with new cartridge and limit set and handle stop and etc.... but that didn't help like I said its only year old. The pump has three settings Auto, Temperature, or 100% I have tried all of them makes no difference. Any Suggestions
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
Did the plumber install check valves at the point of return, (where the return pipe starts to go back towards the pump and tank)? If not the water could flow from the bottom of the tank, back through the pump and affect the water temperature at the shower. Is your house piped with 1/2" or 3/4" water line? I would run the pump for a few minutes, then turn off the pump , and shut off a valve to isolate the pump line and go and take a shower. If the temperature in the shower remains constant as it should then the problem is the pump line and check valves will have to be installed. ( one note: the pump should have been installed with valves to isolate it for when you need to repair the pump.)