Kernel NTP flipping between FLL and PLL modes

[Peter Jeremy]

My 5.x machines are regularly reporting that the kernel is flipping
between FLL and PLL mode (as shown by STA_MODE in syslog messages).
This isn't occuring on my 4.x machines (they typically report 2040
then 2041 and stay indefinitely in that mode).

Any suggestions as to why this is happening?  (And how I can stop
it regularly flipping)

A fairly typical set of syslog entries looks like:
Apr  1 00:15:16 fwall2 ntpd[407]: kernel time sync enabled 6001
Apr  1 00:32:22 fwall2 ntpd[407]: kernel time sync enabled 2001
Apr  1 01:23:36 fwall2 ntpd[407]: kernel time sync enabled 6001
Apr  1 01:40:42 fwall2 ntpd[407]: kernel time sync enabled 2001
Apr  1 10:09:10 fwall2 ntpd[407]: kernel time sync enabled 6001
Apr  1 10:26:14 fwall2 ntpd[407]: kernel time sync enabled 2001
Apr  1 12:59:58 fwall2 ntpd[407]: kernel time sync enabled 6001
Apr  1 13:51:14 fwall2 ntpd[407]: kernel time sync enabled 2001
Apr  1 16:07:48 fwall2 ntpd[407]: kernel time sync enabled 6001
Apr  1 16:59:06 fwall2 ntpd[407]: kernel time sync enabled 2001
Apr  1 19:15:42 fwall2 ntpd[407]: kernel time sync enabled 6001
Apr  1 19:49:48 fwall2 ntpd[407]: kernel time sync enabled 2001

-- 
Peter Jeremy