FVS336Gv2 DNS Issues?

This week, I was trying to get a VPN established on my office's FVS336Gv2 router. After encountering some issues, I opted to attempt a firmware update to see if that could resolve the problems I was running into.

After updating the firmware, I was now able to get the VPN working - well, somewhat (still having some issues but those are relative to my own routing configuration, I think, not the router). However, since the firmware update to 4.3.3-6, I find that the router interface is extremely sluggish (not a huge deal, I'll live), but more importantly, I've introduced some severe DNS issues to the office network.

Periodically, Web sites will fail to load and e-mail servers will fail to be reached. Google Chrome (the browser I have installed on most of the workstations) will usually pause on "Resolving host" in the status bar the bottom of the window, before returning a "No Internet" or "DNS failed" type of message. A refresh or a re-load will usually connect successfully (though a couple users see the error and hit the brakes hard before doing anything else). I find that once I've reached a site (i.e. Gmail or our asset-tracking site), I don't get failures, but sometimes it's loading the site at all that's the problem.

For reference: I have two WAN connections feeding the router (one dynamic, one static IP, our ISP is Comcast Business). The router is connected to a ProSafe GS748T 48-port switch, into which all our servers/backup devices/wall ports are connected. I haven't touched the 48-port switch, not even to reboot it. The router is also connected to an ancient Linksys WRT54G wireless router that has been set up as a wireless access point only (DHCP and all are disabled). The router and switch were installed together. The only thing I've changed that precipitated this (at least I assume!) is the firmware on the router.

The majority of the workstations are set to use static IPs (for predictability), with their DNS server addresses set to those of our ISP. Most of the workstations are running Windows (with a couple MacBooks to boot).

So far I've thrown the following at it:

- Set the router's DNS servers to Google's public DNS, which made no difference (didn't change all the workstations though, just the router).

- Disabled the DNS proxy setting, which makes no difference.

- Reset the router to base settings, which restored it to a clean install of 4.3.3-6.

- Downgraded firmware to 4.3.3-5; the user interface is zippier but functionally there's no change.

- Switched the router from NAT to Classical Routing, to no avail.

- Changed load balancing to use the static IP as the primary WAN with roll-over (didn't really expect anything but I was desperate).

Basically I'm trying to find anything that's different from how I had the router configured with the base firmware - and I didn't really change much outside of network addresses and a forwarding rule for our security camera system.

What is there at this point that I should try next - or that I haven't tried? Should I try finding an even older version of the firmware and go that route? Could there be a hardware issue in tandem with the new firmware?

Thank you!