Router – Linksys RV042 DNS problems

I have a Linksys RV042 router, hooked up in a fairly standard configuration (only using one WAN port – hooked to a cable modem). DNS lookups from systems on the LAN are failing intermittently. This happens even if the computers are manually configured to use specific name servers (not using a proxy on the RV042), it happens with different name servers (comcast, opendns), and it happens with Linux and Windows boxes.

I did some experimenting with tcpdump and wireshark, and it looks like the responses from the DNS server are sometimes coming back with an incorrect port number – using a port (at least sometimes) from a previous DNS request. That is, I can see all of the DNS requests going out as expected, and most of the time the response comes back correctly, but maybe 1 out of 20 times, the response comes back to a port number that's different from the one used in the request.

Is there a known problem with the NAT implementation in the RV042 that would cause problems with incoming UDP responses? Or is there some sort of configuration problem that I can fix?

I've seen a bunch of similar inquiries on various forums, but no useful answers so far…