Well we have also seen this at work, are you using the built in VPN client in XP or Windows 2000?
It also could be your ISP, some isp's like Comcast state if you are going to use VPN you need to upgrade to there pro service.
We use the Nortel client and had to upgrade to 4.65 to fix this issue, look into upgrading your software to the latest version.
We have also found that if a pc goes into sleep mode while connected it will disconnect, maybe the remote pc is hybernating.
Hope this helps.