strange e1000 performance issues

I've got a pair of dual G5s running 2.6.4.  For a second gigE link we 
added in some Intel 82540EM cards, using the mainline e1000 driver with 
NAPI enabled.  lspci reports the bus as 66MHz.  The cards were connected 
together with a crossover cable.

A basic UDP packet-blasting program in userspace using 64KB datagrams 
could only manage 47MB/s of data throughput, using 20% of a cpu.

For comparison, the same program managed 108.9 MB/s with 30% cpu when 
using the onboard gigE ports.

Also, I tried setting a larger mtu using "ip set link dev ethx mtu 
xxxxx".  No matter what value I chose (even setting it to its default 
value, which should have had zero effect) it resulted in the link not 
being able to transfer data.  Downing/upping the link didn't clear the 
issue, neither did unloading/reloading the driver module.  It took a 
reboot to actually get it working again.

Any ideas what's going on?

Thanks,

Chris