REquest timeout every second ping

Hello People
I have a simple WAN link setup for our Melbourne & Sydney Offices.
The link is operational and we have traffic going across it, however I've
found that there are some problems between certain combinations of machines.
For example, when I try pinging from a WinXP Machine located in Mel to a
Win2k machine located in Syd (& vica versa), I get request time-outs every
second ping. However when pinging from XP to XP or 2k to 2k, everything is
fine.
Anyone have any ideas?

Here is a snap of a ping between a melbourne xp machine and a sydney 2k
machine
M:\>ping <sydney machine>

Pinging <machine> [192.168.101.3] with 32 bytes of data:
Reply from 192.168.101.3: bytes=32 time=19ms TTL=126
Request timed out.
Reply from 192.168.101.3: bytes=32 time=18ms TTL=126
Request timed out.
Ping statistics for 192.168.101.3:
Packets: Sent = 4, Received = 2, Lost = 2 (50% loss),
Approximate round trip times in milli-seconds:
Minimum = 18ms, Maximum = 19ms, Average = 18ms


I have the following setup:
Mel LAN<-->Router<-->PRIVATE NETWORK<-->router<-->Syd LAN
The Routers are configured as straight through routers (no firewall or NAT).
Network connectivity between routers is perfect.