I was reading in Run Strong- Editor Kevin Beck today. There is a section on muscle imbalance. There is a diagram of the leg and the muscles (front view and back). It appears that the one I keep hurting is the Biceps femoris.

In the section he says "Hamstring injuries in distance runners are often the result of poor core-muscle strength........ Along with stretching at appropriate volume and intensity levels, certain strengthening exercises- among them eccentric an dcore-strengthening exercises- can, through stabilization, prevent or alleviate hamstring pain."

I guess I see what I need to start doing on a regular basis. When I was at peak shape I was doing core exercises 3 times a week and stretching daily. I would like to get back in the habit of doing that but normally I do them for a couple of weeks and then forget only to have another long lay off of doing them. It's a bad cycle I wish I didn't do.