hmmmm, as someone with major neck problems I certainly understand your frustration.

What comes to mind is this - and I don't even know if this makes sense but I will toss it out there anyway. I wonder if a smaller diameter bar is causing you to grip more firmly than you normally do. I can see this leading to other muscles becoming tighter than usual, which could travel upstream and cause more tension in your neck. I know that in my situation, at least, I've learned that very small things CAN cause neck symptoms for me (and other things that SHOULD don't, so go figure).

Hang in there and keep us posted!