This may not even be close to what is the problem, but humor me please [img]/ubbthreads/images/graemlins/grin.gif[/img]
I have an older Canon A2, it got to the point where my on camera flash would not come up. Understanding yours does come up, the problem may still lay here.
When using heavier(cheaper in my case) hot shoe flashes there is a tendancy for the flash to put adverse stress on the hot-shoe itself when shooting vertical formats, because the flash is hanging on the camera instead of sitting on the camera. I don't know if the D30 has any different switches under the hot-shoe then the A2 but I fixed my problem by following the instructions in the following link....
On the A2 there are two micro switches under the hot shoe that as the hotshoe as stressed laterally willl no longer return to the released position, thus fooling the camera into thinking it has a flash mounted and not letting the on camera flash pop up. I understand yours comes up but there may still be something there that effects your sync.
If your not real mechanically inclined you may choose to leave this to someone else.
Doubt if it helps but good luck with it anyway [img]/ubbthreads/images/graemlins/tongue.gif[/img]