Amount of time to stable pH?

[smokyjoewood12]

I have the Milwaukee MW101 pH meter, its a great meter, but today I had some strange pH readings during mash, it was higher than expected and it seemed no amount of acid would bring the pH any lower. So after the brew I calibrated the meter in 4.01 and 7.0 solution, which proved it was already calibrated. Rinsed with distilled water and checked the pH of my tap water to see if it was 7.7 as it was when I had it checked by a lab. I put the calibrated, rinsed probe in 2L of tap water and it seemed to stabilize at 6.67 @ 20c which seemed way too low for my water, and then over the past 3 hours has been slowly rising to 7.57 @ 20c where it is now and seems to be stable. 3 hours!? I don't understand, it stabilized very quickly in the calibration solution so it doesn't seem like the probe is faulty, I just don't know what would take it so long to stabilize in the tap water... Any insight?