Things are back on. No MIDI dropping out mid-session!

Only one reset needed mid-way through after I let Cubase sit for 20 minutes.
Changes:
Removed Logitech USB receiver
Disabled LAN/ethernet in device manager
I wasn't in the mood to try one and then the other. My bet is the Logitech receiver. Oh, and here's a thing I learned from an EMU tech note, very handy:
Disable Legacy USB support in the bios!
That does NOT mean you can't use USB keyboards and mice. It DOES mean you will need a PS2 keyboard handy for bios and startup.
I've been measuring my systems latency with the excellent DPC spike checker. Legacy USB support at the bios:
Enabled:
80 us (idle peak)
Disabled:
50 us (idle peak)
Was very surprised! I've never had a desktop go that low.