Back pains when cycling

Question
When cycling (even a low speeds) after about 15 minutes my lower back gets sore.  I've tried lowering/raising the handlebars and seat but I still get back pains.   I'm about 6ft tall with a long back (little longer than average) and my bike is a diamond frame one 19inches.  Anything I can do to stop the pains?

Answer
Hello James,

Is it only your back which is sore, and is this going away if you stop cycling?
Are you overweighted?
Then this could't be the problem, stonger your musles, see for good exercises a fyschiotherapist or even a good gym

Does it hurt like a nervepain in one or both legs?
When this happens go unmediately to your doctor.

Further I can only tell to go to your cycleshop which has maybe more experience with the correct measerements from you or your bike.

Regards,

Michael