My pH meter is nervous around wort

Anyone else have this problem? I use an econ pH meter but it seems reasonably reliable . . . until I test my wort. When calibrating it will settle down in less than a minute and is easily adjusted. Once adjusted it appears to be steady.

Then I pull a wort sample and cool to room temp. Initially the reading is sub-5.0, but it gradually creeps up. Last brew day had a computed mash pH of 5.45. The initial reading was around 5.0 but over the course of the mash it ended up in the mid 5.4 range . . . but this took 45 minutes. When I place it back in the calibration solution it quickly reads out the correct reading. Meter has always been rinsed with distilled water and stored with storage solution.

Can anyone else relate to this or have a favorite model that gives reliable (and quick) readings?