I used to have the same issue with the Tamron 17-50 and the K-5 (and my K-x DSLR, fwiw), although the under-exposure was never more that 0.5 to 1 stop. However in a recent shoot involving literally hundreds of shots with the K-5 and the Tamron 17-50 in daylight (and using flash too), the problem wasn't there at all. I was shooting the exact same locations as well that I used to have problems with last year.
All I can suggest is to perhaps update to the latest K-5 firmware, and maybe try centre-weighted metering for a while. Perhaps also check under the camera menu options that you haven't set the AE metering to follow the AF point, or something like that.
Another suggestion would be to open up a problem image in Photome (see image below) and see what the AE point graphical display shows about the LV of the scene. And maybe also just generally review the EXIF data to see what the camera is recording for your problem images.