You mention shoulder problems as well.
It sounds like you are a good player with overall sound technique, but over time little changes can creep in that can be harming your shoulder.
Are you doing anything that could be helped by reviewing these shoulder videos:
Your serve technique doing more harm than good? http://www.youtube.com/watch?v=GgdXawklcZk
Preventing Rotator Cuff Injury http://www.youtube.com/watch?v=lTRvxaBMh8s
Rotator cuff injury http://www.tennisresources.com/index...v=1&vidid=3712