I'm currently using a D80 usually with a 24-120 f3.5 ED VR and appear to be having some exposure problems. Another photographer I shoot around is using a Canon EOS 1D Mark II N and usually gets much better exposed shots than I. He uses a flash more than I do but even my flash shots aren't exposed correctly all the time. Our conditions are extreme. We have thick dust, rain, snow, bright sun with dark shadows and very fast moving subjects at close range most of the time.
I'm letting the Camera select the exposure and setting the flash on TTL most of the time. I'll bump the flash up or dow a couple of stops considering how dark the shadows are.
Oh, by the way, if you are shooting things like motor cross where there is a lot of dust and mud flying in the air, and you are shooting on Aperture priority, you can expect your photos to be overexposed.
I had this issue shooting the 4x4 Club's mud bogs last week. Once the mud started flying the camera would overexpose the images because it was metering all of the dark mud.
You might try getting a good meter reading before dirt starts flying and stick with Manual, checking your meter once and a while as the lighting changes.
I had a few badly overexposed shots at the last race. I'll have to look back and see what my settings were. I've never uploaded pics here so this is the first try.
I would suggest you D3 or D3X for shooting sport and action... (take a look at www.kenrockwell.com) yes both bodies are expensive but faster than D700... why dont you get D200 - 5 fps or D2X - 8 fps? they are fast enough too(and much cheaper), also they dont tend to blow up highlights as D80 does...better metering and sensor resoliution especially D2X...