I don't know what to say about the lockups. My first reaction was that you should change to all Nikon equipment, but since you used your lenses for so long without problems that doesn't seem to be the answer.
Regarding the flash units and overheating, you should go to RobGalbraith.com and read his review of the SB-700. It gives a good overview of the overheating problems with the SB-900, and then tells you how the lower-priced SB-700 fixes the issue (it doesn't heat as much in the first place, then slows down rather than going until it MUST stop). I agree with him that the SB-700 is easier to set, and seems to give better exposures.
I have had SB-900s for a couple of years now, and they've stopped working in the middle of shoots on me. The SB-700 gives up a half stop or so of light, and does NOT accept a battery pack, but its lower price, lower weight, smaller size, and tendency to never overheat are very endearing. In fact, I have followed Galbraith's lead and switched to using the SB-700 exclusively for on-camera flash work.