OK here are some test results.
I installed Cubase on this P4 so that I could test without using the network card, and used the offending midi sequence from the Host PC (ie. using the P4 as a host, rather than a slave).
a) With a buffer setting of 512, the Delta card played flawlessly on PCI slots 2, 4 and 5. Slots 1 and 3 broke up.
b) With the Gigabit card connected to PCI slot 3 (but no network activity), playback was still flawless.
c) When I play the sequence from the Host PC to the slave P4 viaMidiOverLan, bad audio breakup occurs.
So the problem seems to be caused by the Network (Realtek RTL8169/8110 Family Gigabit Ethernet).
I'm only using 6 midi channels with modwheel data, slow note tempo, nothing frantic - should be a piece of cake for the network to handle.
Any suggestions?
Thanks,
Mike.