G10 occasionally under exposes, live histogram moves to the left

I have a G10 and it is driving me round the bend. When using the G10 in program mode it sometime underexposes. I can take two shots of the same scene within seconds of each other and one can turn out underexposed and the other OK. The difference in exposure of the two shots can be over 1 stop!

Please see these two shots that show the problem well. Note that in both cases I had exposure compensation set to -1/3 to try to stop highlight clipping.

In the underexposed image I noticed that the live histogram shifted to the left when I half pressed the shutter. Does anyone else see that behaviour? Does anyone have an explanation for that behaviour?

I should add that the lighting was pretty contrasty with the sun high in the sky to my right, maybe that was affecting something.

All help and advise appreciated. Steve.