It looks like piggy-backing 1 NAT subnet off of another was the problem (a crossover cable running from the main router to the WLAN input port of another, which was doing its own DHCP), which admittedly was a terrible setup from the get-go but I was lazy and it never occurred to me that hey, maybe my shitty little $45 Netgear from 3-4 years ago could probably be set up as a bridge too and this would've never happened. It was also hard to troubleshoot since the stream was the only thing messing up (XBLA, Skype, IRC never dropped out on this setup but as Bobby said Shoutcast is particularly fussy and high maintenance).
We now have a Linksys E2500 as the mothership and the old Linksys WRT54G running purely in bridged mode; everything seems to be great. The show computer ran off the bridge tonight, but we did have a long cable prepared to run it directly to the main router during break if needed. It also gave us an excuse to update the firmware on the WRT54G to something a view versions newer than what we had, even if it was still from 2009. The E2500 is also dual-band broadcast and .n WiFi protocol so we needed that anyway.
There are still other upgrades we can do, like switching to Gigabit NICs in all our machines and using Gigabit cabling but we can earmark those for next year and after we move.

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We also ran this last night:
Ping using 256 byte packets - 6pm to 9:45pm
Ping statistics for xxx.xxx.xxx.xxx
Packets: Sent = 12930, Received = 12918, Lost = 12 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 86ms, Maximum = 188ms, Average = 91ms