a. Chris Harmon (of Newer Technology) wonders if this is connected to the "famous 9500/200 problem." In that case, not interleaving the memory corrected the problem. The ultimate cause turned out to be a problem with the memory controller on the logic board. However, Chris writes: "Using memory built above the JEDEC standard also fixes the problem. Of course we offer such fine memory ourselves."
b. Andrew Harrow writes: "I have seen a similar problem with the 7200/120. With or without extra RAM installed, on some 7200/120's the Ethernet activity would start to copy, and then run somewhere around 110 baud. This problem was rectified by swapping out the logic board, for an AppleCare replacement.
I have also seen trouble on the 7200, 7300, and 7600 when you mix DIMMs that have 2K and 4K refresh rates. If the refresh is not the same as the pre-installed Apple RAM, the computer will start up, not make the startup sound, and hang. If you have, for example, a 1710 monitor, it can be quite difficult to troubleshoot, because the monitor will not sync on and will likely not power-up.'
To get around this problem, if you are going to use non-standard refreshing DIMMs, rip out the Apple ones, and keep them all the same.