Grumpy,
I've got a F75-LTD, A CZ20 and a CZ6A. I've used the CZ20 quite a few times to go dirt digging in the rain. The CZ's are dead quiet hunters in places where the F75 gets chatty due to EMI. Sensitivity setting of 3 or 4 to not false is not unusual on a CZ, although I can run a bit higher on the two older units that I have. While I can hunt through chatter with the F75-LTD, the CZ's falsing a lot due to high sensitivity settings are not friendly to hunt with. You want a CZ that hunts quiet except when you are on a target, so adjust the sensitivity accordingly.
I guess you checked batteries and battery connector just to rule out the batteries as a potential issue?
If it truly is getting affected by EMI where the F75-LTD is OK, I'd seriously think about sending it in for a checkup.
Hopefully some one on here with a CZ21 can give more thoughts on the sensitivity setting they use with a CZ21.
Cheers,
tvr