Pentax MZ60 underexposing

I picked up a new Pentax MZ60 a few weeks ago and have put a few rolls of
film through it. I have noticed that some of the shots are terribly
underexposed. I have been trying to find some rhyme or reason to it, and it
seems to be the shots that are outside but in shadow that are more likely to
be underexposed. I am mainly using it with the sigma 28-80 lens that it came
with, as well as with another 28-80 lens (tokina brand) that I have, and it
seems to do it with both. While I have shot mainly on the green setting, I
have used it a bit on sports, macro, landscape and Tv and Av modes. The
underexposed shots are mostly on the green setting, but have also occurred
on the other settings. I even took some family shots on the tripod using
the timer (and with the cover over the viewfinder) - as is my practice I
took a few in case one of us was pulling an odd face. Out of the 3, one came
out terribly underexposed.
It sounds to me like the camera has an intermittent light metering fault,
but could it be something else causing the problem? I thought it might be
not stopping the lens down correctly, but I think if that was the case it
would over-expose, not underexpose.