OK, so you have a PIX, a router, an entire WAN, and another router in between your PC and the AS400, yet you decide that the PIX is the cause of the problem. What has led you to that? And if you're pinging with packet size of 1500, why would the PIX need to fragment if the PIX MTU is set to 1500?
The cause of the problem is most likely that you're blocking Path MTU Discovery from working properly, by not allowing the ICMP packets back in through the PIX.
You'll probably find one of the paths in the WAN has a lower MTU, and so that router sends back an ICMP Unreachable back to your host. Your host should then fragment the packet down itself so that all fragments can make it to the AS400. The PIX however, will be blocking this ICMP Unreachable from getting back to your PC unless you have explicitly permitted it.
Add the following to your PIX:
access-list inbound permit icmp any any unreachable
access-list inbound permit icmp any any time-exceeded
access-list inbound permit icmp any any echo-reply
access-group inbound in interface outside
and see how that goes.
Read up on http://www.cisco.com/warp/public/105/38.shtml#pmtud_fail for some further information.
