> WORKED:
> (LAN switch) <--> WRT54GL (OpenWRT Kamikaze r16206)
>       |               |
>  [MythTV Backend] [MythTV Client]
>
>
> Too choppy/jerky to watch:
> (LAN switch) <--> [M0n0wall] <--> WRT54GL (OpenWRT Kamikaze r16206)
>       |                              |
>  [MythTV Backend]               [MythTV Client]
>
>

If I understand correctly, in your first setup there is NO monowall at 
all ? What purpose has monowall in your setup ? (I'd suspect the captive 
portal... did you try without captive portal)
What are both IP ranges, I'd suspect that both are a private range... 
did you uncheck the "Block Private networks" option on the WAN side - if 
so ?

Regards,
Michel