Trouble with VPN traffic passing through Juniper firewall

I have a VPN tunnel between a Cisco ASA 5510 and a PIX 501.  The PIX is at a client's site and lives behind the client's Juniper firewall.  Recently the tunnel has started dropping off and not re-establishing.  I did some troubleshooting with the client and it appears that their firewall starts dropping ISAKMP packets bound from the PIX back to my corporate office.  My Client hasn't changed anything recently, and his firewall rules don't restrict outbound traffic, but strangely enough, as soon as he put explicit rules in place to permit outbound ISAKMP, the tunnel started working.  After sitting in that state for over a week, I thought we would be ok, but then it started doing it again.  The ISAKMP rules were still in place, so my client removed those rules and re-added them and it started working again.  I just thought I would throw this out there to see if anyone else has run into a similar problem with a Juniper firewall between two Cisco VPN end-points.

Thanks,

CT