Pentax k100d metering problem

I have a problem with my Pentax K100d that I would be gratful if
anyone could
offer any advice on.

The problem is that in any mode requiring auto exposure, the camera
massivly underexposes to the extent where output is just black. For
example, in low light conditions where I would expect shutter speeds
of say 1/20 or slower, it is selecting 1/500 - 1/2000. Intermittently
however it works correctly. Manual exposure works fine.

This occurred a while ago and it was sent back to pentax who seemed to
fix the problem (albeit they kept the thing for 2 months). The
service docket said something like exposure reset or re-calibrated.
After that things were ok again for a while but the problem has
started again.

The camera is still under warrantee (just) and so I intend to send it
back again but my concern is that pentax will do a temporary fix again
but that the camera has a more serious fault somewhere. Can anyone
suggest what the problem might be? Should I be insisting on a
replacement? Can the exposure be user reset/re-calibrated? many
thanks for any thoughts or suggestions.