help - high cpu usage for serial connections from com port

well, as the title says -
very odd - just comparing notes with some of our US engineers who are over on a training course
we'd noticed that when we're connecting a serial connection from an on-board com port on a win2k server to a vxworks server diagnostic port it's using 30% cpu for the system process on the 2k server.
does this regardless of the tool in use
using a pci rocket express serial card - no problems, also not repeatable using a usb diagnostic serial adaptor
Has anyone heard of this before as it's now bugging me!