lower back/groin ache

[Broughtonblue]

after about 20 minutes ive started to get an ache in my very lower back, about at the same level as my hips. towards the end of my regular 2 hour circuit the ache has increased through to my groin. during the ride i can relieve the pain slightly by standing on my pedals, this gives me a couple of pain free minutes. at the end of my ride after about 5 minutes the ache completely goes. obviously it has something to do with my set up, ive tried highering the saddle, moving it forwards/backwards and adjusting the tilt but with no effect.
i ride a giant scr2 and have had it for about 3 years and cant recall having this pain before. if i go out on my scott p4 hybrid i dont get any problems.
just hoping for any help/tips if possible

 

[Broadside]

My top tip for lower back pain during a ride (I get it quite a bit on 2hr+rides) is to stop and gently stretch your hamstrings, find a gate or road name sign to rest your heel on and gently stretch it through. This makes mine disappear, it is to do with tight hamstrings pulling on the pelvis and rotating it downwards causing strain on the muscles attaching to the top of the pelvis, i.e. your lower back.