I have recently had two bathrooms renovated and installed Moen Posi-Temp one handle shower faucets in both. http://www.moen.com/adler/chrome-posi-temp-r-shower-only/_/R-CONSUMER%3AL82691
http://www.moen.com/eva/brushed-nickel-posi-temp-r-tub-shower/_/R-CONSUMER%3AT2133BN

After the water has run hot for about 3 minutes, the temperature gradually drops. I am able to turn the handle further to the left (hot) which increases the temperature again. It continues to get cooler and I get to the point where I cannot turn the handle any further. If I turn off the shower and wait about 10 minutes, the hot water comes back, but then gradually drops again.

I have a 50gal electric hot water heater and have plenty of hot water when using other faucets around the house. Before the new shower faucet was installed the water in this shower ran perfectly. Also, when using the shower, no other sources of water are running.

I contacted Moen customer service and they said it must be bad cartridges and sent me new ones to install. I thought this was a little weird as both bathroom shower faucets were purchased at different times and from different retailers. I had my contractor install the new cartridges and the problem continues. He flushed the lines and increased the max temperature range on the valve. I also tested the temperature of the water from the bathroom sink. If the sink faucet is run hot only, the temperature is HOT so I can't believe it has anything to do with my water heater.

Please help me figure out what the issue is... I can't stand these cold showers!

I am having the same problem. Did you figure it out?

Yes! Turns out it was the water heater after all. One of the elements wasn't working so only some of the water in the tank was getting/staying warm. We replaced the heater and now both showers work a dream.