My D300 is overexposing?

Hi,

I have been shooting mostly interior and bright daylight conditions with very few metering problems so far with my D300, however today I took my camera with the new 17-55DX for a short hike, at first everything was all right and the camera exposed very well,

but when I stepped into the shady areas camera suddenly started overexposing by 2/3~1 stop. I had the LCD off so I didn't figure this out until I got home and downloaded my photos, here are two overexposed examples with histogram, I had about 30 photos that were overexposed!

looks like at some point towards the end of the hike things came back to normal when light levels fell even more, everything from here was exposed OK

I was using matrix metering for all shots. I have seen some overexposure threads but I never read them since my camera seemed to be OK but now I am worried, are there any known solutions to this problem? Are all D300s like this or I just got a lemon?

Thanks,
Arash