Looking into ur problem, I would just want to know some details:
Did u establish an ipsec tunnel b/w the VPN concentrator and the other end .If yes, then
you would need to run debugs on the router to see what is happening.
the TCP Maximum Segment Size(MSS) for PPP over Ethernet/ATM cannot be over 1452.
Lower the inside interfaces' MSS to 1400 on both end and check it out.
The reasons for this pblm may be:
You can also clear df bit and see if that works, by clearing the df bit the router should fragment
the packets if needed.
Also, there might be a device which have mtu size adjusted to 992 on your network, this might be on
the router/firewall/workstation. Please see if this is the case.
You can refer to the following document for more information on MTU and MSS issue: