The valve stems are prone to failure. What happens is the polyurethane seal pushes out of the brass body and all sorts of weird things begin to happen. I've seen wild velocity swings first hand. Typically if you keep using it, it eventually loses the seal and all the gas evacuates.
 I also agree with Anthony's point, I've heard of the stems pushing out as well. That's more likely to describe the problem you're having. [/edit]
Either way, you can replace it with a QB valve stem or one from Roy at Mountain Airhttp://mountainaircustomairguns.com/faq/
The Teflon version is preferred if you will be using it with CO2. It can be difficult to get Delrin to seal reliably at CO2 pressures.
Regarding the hammer spring adjustment, I'm a little surprised the same setting worked satisfactorily for CO2 as well as HPA at roughly twice the pressure. My guess is it was wasting a lot of CO2.
These guns flow like mad so your 15-17fpe target should be easy to achieve on HPA. You mentioned you are considering a bstaley buffer. That works quite well in these guns to bring the power level down to something more modest like that and achieve longer, flatter strings. Looking back at one I set up for a friend, we went with a 14.5fpe tune (middle curve) that yielded 30 shots inside of a 4% ES.