Hi,
Do you have ICMP Inspection enabled?
If not, add
fixup protocol icmp
fixup protocol icmp error
If those dont help, you could consider adding these to the external interface ACL
access-list permit icmp any any time-exceeded
access-list permit icmp any any unreachable
This document might also help you with troubleshooting and configuring
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_tech_note09186a0080094e8a.shtml
- Jouni