IPF Generating ICMP unreachable?

Since going to IPFilter 3.4.31 I have noticed that connections to some
hosts stall. They get some data and then stop in the middle. This is
often reproduceable but not exactly the same each time.
Checking the packet sniffer, I am seeing ICMP destination unreachable,
host unreachable messages coming from the firewall/nat machine running
IPFilter for a handfull of packets in these connections. No ICMP messages
are received from the upstream on the router, so they are being generated
by the machine. I'm not sure how the reachability would be changing so
rapidly.

Any ideas why these would be generated?