Hamstring Tightness

Just one suggestion which may help?
Have you tried any massages? that might help loosen the muscle a bit?

Good suggestion - increased blood flow to the muscles helps a great deal with flexibility.
Also, and I'm just playing devils advocate here....are you really doing a diligent a job on stretching BEFORE and AFTER each and every workout? And do you set aside time for actual stretching workouts, not just stretching sessions? I have to ask this because I have never once observed a person in the gym doing a good job of stretching. It's always half assed, without fail. So I just thought I would throw that out there..I'm not saying you're half assed...truly I'm not!

Do you know what caused the problem to begin with? Was it from running or cycling or some combination? If your saddle is too high that can cause hamstring problems.

I pulled the hamstring during LA Marathon 2003. Runner did a full dead stop right in front of me. I side stepped him, planted my foot and pulled the hamstring. Official saw the incident and DNF'd the runner and pulled him off the course. I have had problems with it ever since. I do diligently stretch before workouts, however, I have to admit, I do not do stretch workouts. Someone asked if I did this on bike or running. Initial injury was while running, but it can be aggrevated by running or cycling. (I don't aggrevate the hamstring during swim workouts.)