> Pings to/from the C650 (SONIC) to/from any other machine take between
> 5-9 seconds!

This sounds like you're losing interrupts.  Do you have the latest
grf_iv.c?  (version 1.19)

> I have things set up exactly the same on the IIcx and the C650.

I assume that's "except for the IP address"  ;-)
Have you tried cold-booting the 650 w/o extensions and w/o configuring
the network stuff at all on the MacOS side?  The Q610, it seems, works
much better if MacTCP hasn't been using the ethernet...

-allen

-- 
              Allen Briggs - end killing - briggs@macbsd.com