Hi,
all of the pre-1965 dimes including mercuries, that I have found have hit in the 24 to 28 range depending on depth and mineralization. I don't understand your assumption that they would be in the 30s.........according to what or who? Surface clad quarters can hit 30 or 31 and so can really deep copper pennies, clad dimes, silver dimes, clad and silver quarters depending on mineralization levels. All DEEP (6"+ in my bad dirt) medium high and high tone coin sized targets using default 5 tones in Park 1, sensitivity 18 to 20 will have somewhat jumpy numerical target IDs and associated tones. I have seen 6 to 8" copper pennies hit at 24, 25 in one direction and hit 28 to 30 at 90 degrees. As long as these targets show up as 3 to 5 depth arrows, hit medium high to high tones in both directions with no adjacent iron responses (check by using the horseshoe button to temporarily remove discrimination) and they appear to be coin sized objects, (check with pinpoint sizing or raising the coil) I'm digging. If I get one or two negative numbers or iron grunts with the horseshoe button engaged I will probably still dig. Usually deeper iron targets will have medium to high tones and numerical target IDs in one direction and repetitive low to negative numbers and iron grunts at 90 degrees.
The only coin targets that I have dug that hit above 30-31 were two Kennedy halves stuck together in the same hole, two or more clad quarters in the same hole and golden one dollar presidential, Susan B Anthony and Sacajaweas at depth.
You might want to recheck your presumed target ID numbers for the Equinox.
Also, default Park 1 and the other modes too have default sensitivity set at 20. From what you have mentioned about varying mineralization in your area, 20 might be a bit too high and could be causing coil knock, falsing or detecting of larger natural iron particles in your soil, sand and moving water.
Jeff