Question : TCP connection dropped -- 'Establish TCP Abnormally'

Hi - this is my first question on EE
I am hosting a Citrix MF server behind a Netgear FVS318.  Clients connect from multiple locations around the country.  One location has recently (last three weeks) begun losing connections periodically throughout the day, i.e. all users connected get dropped at the same time.  They are not able to reconnect for 3-5 minutes (they receive a 'No Citrix Server configured at this address' message) when this happens but they still have internet access and can ping my IP address.  After the interval they are able to reconnect and work normally until it happens again.

The firewall log reports:
Fri, 05/13/2005 04:14:55 - TCP connection dropped - Source:XXX.XXX.XXX.XXX, 1731, WAN - Destination:YYY.YYY.YYY.YYY, 1494, LAN - 'Establish TCP Abnormally'

During these occurances, other remote users are unaffected as are local users also behind the firewall.

The location suffering the problem is using a T1 connection.  Their LAN connects to the T1 via an inexpensive Barricade router.

I suspect the problem is either with their router or possibly the ISP, XO Communications, who supplies both my T1 connection and theirs.  There have been no hardware/software changes at my location and all other remote locations are not experiencing any problems.

I guess my question is what type of problem generates the log message 'Establish TCP Abnormally'?  Where should I concentrate my troubleshooting efforts?

Thanks,
OM Gang

Answer : TCP connection dropped -- 'Establish TCP Abnormally'

First thing I would check is, client side PC for Virus/Adware/Trojan.