Niggling Hamstring for years

hey,

I injured my hamstring while playing football probably around 6-7 years ago now.  I just felt it pull while sprinting.  I didn't have any bruising but I also never iced it or anything and to be honest, I didn't do much to try and repair it and to let it heal.

Ever since then, if I run unexpectedly without a proper streth and warm up I feel a sharp(ish) shooting type pain in my hamstring area.  I do go to the gym and do exercise my legs, curls, reverse curls, squats, deadlifts etc with no probem.

Even if I feel I have warmed up properly, as soon as I have to sprint, i feel that pain again.  It's not that it's painful, it's more of a discomfort.  I then end up having to take time out and stretch/warm up again! 

I can run on treadmill with no issues however the sprints I really only do if I'm outdoors or playing football.  Sharp movements such as performing a backhell at footy will cause the same pain if not warmed up properly.

The pain is most apparent however when I'm on the punchbag.

Is this something i will have to live with now or can I still eradicate this problem?