Tom,
What are you doing using vinyl down there. Don't you know you're not supposed to play records wet? ;-)
We do have good AC but not central, just thermostatically controlled window units. The house isn't as tight as a newer one would be and southern New England (CT) weather presents challenges because it's always changing. A full four season climate with day-to-day swings of as much as 40 degrees. Last week it was 98 degrees with near-Pontchartrain levels of humidity. Two days later the daytime high was 55 and raining. Two days after that it was 80 and dry (ah!). In the winter temps swing from 40 to near zero and the air can be throat-scratchingly dry. Your climate, while warmer and wetter, is more stable and "should" present a smaller range of challenges, hurricanes excepted of course!
BTW it's not just ZYX or the UNIverse. Every MC cartridge we've used has reacted similarly. Elastomers (in the suspension) respond strongly to temp and humidity changes. Shelters, Ortofons and Koetsus behave like this, with individual variations in the numbers of course.
I no longer bother trying to find the actual mistracking point very often, we know the sound well enough to dial it in by ear. For a new cartridge or for those new to this technique however, the method I described above is reliable and repeatable. It also teaches you alot about how your setup is behaving, and it's vital to have VTF optimized just above the mistracking point before attempting to dial in parameters like antiskate, azimuth or VTA.
