shower hot water

When I turn on the shower, the water warms up, runs hot for 15 seconds or so, and then cools off. The line going into the valve is hot, so I guess the valve has a problem. Can this be fixed without replacing the whole assembly? Its an older one handle system. I haven't been able to find a model# or even a brand name.

You can post a picture....it will help someone figure out what you have. When replying, scroll down a little to the manage attachments block.