Check your TCP segment size. To find out what it should be you can try the following.
From a PC at site A, ping a PC at Site B. You need to set some ICMP switches though.
C:\>ping 10.10.2.100 -f -l 1500
The -f is set the Do Not Fragment flag, the -l is the send buffer size. Continue to do this each time decreasing the packet size until you have a successful ping. When you find that number, that is your maximum TCP segment size. You can then set that on the router. Any packet that is too big, the router will respond to the sending device saying it is too big, send a smaller size. It will do that until the packet is less than or equal to the number you set. In most routers the largest size you can set is 1460, so that might save you some time. To set it in the router, go to the tunnel interface and enter "ip tcp adjust-mss
HTH and please rate.