HI GB, Thanks for the quick response.
1) I tried the search term you recommended and found a few entries. Overall there didn't appear to be much consensus on what was causing this problem. I saw some reference to ground balance, sensitivity, and recovering speed playing a role but nothing definitive. For context, my problem seems to occur regardless of ground balance level and i typically run with sensitivity between 18 and 22 and recovery speed of 3 (equivalent to 6 on the EQX 800).
2) "Periodically" was not the correct choice of words. I should have said "intermittently". I can't point to a specific correlation besides the approaching cars that I mentioned in the original post. I will try the 15 kHz switch next time and see what happens.