Every so often mine doesn't work either!
If the tap water is 8 ish PH and Alk of 100 and then adding PH down your getting an 7.6 PH and alkalinity of 120, something is wrong with the readings. Both readings should be affected by the PH down in the negative direction and I can see PH possibly drifting up due to the high nature of your tap water but not the alkalinity. Are you reading PH and alkalinity with a high chlorine reading? Never take any readings when the chlorine is over 3 PPM, at least I don't ... I want to make sure my readings are as accurate as possible.
If not, my suggestion is to use some more PH down, wait overnight, test in AM and see where it is. If it's still high add more, test when you get home and see where it is. It sounds like to me that you water is trying to get back to it's natural state and you need to treat it to get it down. It may take some time treating it before it stays for a while, your alkalinity may become lower than 80 but I would say don't worry about it unless it swings wildly (which it seems to be doing anyway).
