Let us know if the SEF coil fixes the issue. Wouldn't be the first time a coil went bad. If that don't work try a MR.
As far as your explanation of severe ground unstability, i occassionally get that out of nowhere while detecting a site. Normal then in some areas it acts up.
Nothing seems to calm it down except lowering sens. levels or going into AUTO sens.
Then as i move into other areas of that site, out of nowhere the thing calms down to normal. If i return to that problematic area, it acts up again. Happens with all my coils too.
Either its a narrow band of EMI or more likely highly mineralized ground patches or a gremlin in the eTRAC box. In your case, looks like the ground has something to do with it.
Usually happens on hot sunny days. Might even have something to due with thermal parameter changes that shift the coil parameters slightly out of typical enough to the point the eTRAC can't cope with mineralized ground patches. Could even be the sun's radiation, who knows.
There's a reason some coils are white, they reflect heat and other types of radiation from the sun, they run cooler in hot weather.