Milwaukee pH Meter - Help!

[Myka]

I have a handheld Milwaukee pH meter. I have calibrated it with the fluid. I've had it for a few months, and used it a few times. It says you should let it sit until the numbers quit changing, but the numbers will keep changing all day. It slowly loses 0.1, then another, then another. It doesn't go up and down, just down 0.1 at a time, and about 60 seconds in between or so. I let it sit all day once. My first reading was 8.1, and it fell to 6.1 over a couple hours. Wtf? Is this normal?

So...how long should I let it sit before I take the reading?

I'm just re-calibrating right now since I have to open a packet of the pH 7.0 as I notice the fluid in the cap has evaporated. I notice that at first the meter said 8.0, and it took about 60 secs for the meter to get to 7.0. After three minutes it's at 6.9, so I adjusted it to say 7.0.