jamesdak 

I'm trying to work out the same type problem. History of lower back problems to include ruptured and herniated discs. Started cycling last year with no problems and them about a month ago I started having pain. The onset coincides with my increasing my average cadence to around 85 rpm. I also had been traveling for the month prior an used a back-up bike with a shorter reach. So I think mine is a fit problem and am considering shortening the stem.

But to keep from permanent injury I'm seeing my Dr. who is also a cyclist. Hoping he can refer me to a specialist type PT that deals with cyclist to work out my problems properly. Oh, and I am working on my stretching to ease tensions on the back. I worry about core exercises since even a simple act of sitting down can sometimes cause things to slip out of place and cause pain.

You may want to get your physician involved also to properly fix your problem.