Monitored router shown as flapping

Hi all

I have a Cisco router at a remote location which we connect to via its
HSPA+ (3G/mobile data) network interface.

Xymon shows this router (and the kit behind it) as flapping.  I've set
a ping going from the Xymon server to this router and left it going
for approx 20 minutes.  In this time Xymon showed it being
"unreachable" however no pings were lost from the CLI.

We have many routers with this setup and these other routers are not
experiencing this problem.

The server is running RHEL 6 + Xymon 4.3.23 RPM from Terabithia.

Any ideas on this?

Thanks

CC