diff -up ntp-dev-4.2.7p79/ntpd/ntp_loopfilter.c.orig ntp-dev-4.2.7p79/ntpd/ntp_loopfilter.c
--- ntp-dev-4.2.7p79/ntpd/ntp_loopfilter.c.orig	2010-11-08 10:31:04.849887940 +0100
+++ ntp-dev-4.2.7p79/ntpd/ntp_loopfilter.c	2010-11-08 17:50:45.741056405 +0100
@@ -729,6 +729,12 @@ adj_host_clock(
 		adjustment = clock_offset / (CLOCK_PLL * ULOGTOD(1));
 	else
 		adjustment = clock_offset / (CLOCK_PLL * ULOGTOD(sys_poll));
+
+	if (adjustment + drift_comp > 500e-6)
+		adjustment = 500e-6 - drift_comp;
+	else if (adjustment + drift_comp < -500e-6)
+		adjustment = -500e-6 - drift_comp;
+
 	clock_offset -= adjustment;
 	adj_systime(adjustment + drift_comp);
 #endif /* LOCKCLOCK */