Toilets, Sinks, Showers, Tubs and Disposals - Moen hot water problem

I have an oil boiler that heats my house and also provides hot water. I have an amble supply of truly hot water to all sinks and tubs save one. Just recently, I began to have a problem with the hot water in the shower/tub (this is a one handle moen faucet assemly.) Up to now, this faucet worked well, now the hot water comes on very hot for about a minute and then pretty much only cold or tepid water comes out of both the tub and shower. I have some plumbing experience, but do not know what to examine on this--no idea what may have happened. Help!

The first thing that comes to mind is dirt in the valve, the second is that the cartridge is defective. It depends on how confident you are in your ability to work on this. This is what I would do, shut off water to the valve, remove cartridge, check for dirt and flush out valve body, if theres no dirt you may have to replace the cartridge. You will need to know exactly what valve you have to get parts, or take the part to the plumbing house or big box store. This whole operation can be a little tricky. Lots of luck.