I have similar issues with my Asante MacCon SE/30 card. It passes all the diagnostic checks from the troubleshooter program and checks out with the ADLS program, but TCP and AFP connection is spotty.
Also being forced to run a AUI - 10BASE-T dongle irritates me. I believe Bolle moldy has breakout boards for those that do have 10BASE-T jacks.
edit: Those 20MHz oscillators on the digikey page linked on page 1 are discontinued. I found a good match on mouser: ECS-200-18-4X-CKM. Same 20MHz, 4.88mm lead spacing, slightly smaller package, same capacitance (18pF), tigher tolerance.