Vista - Intermittent loss of printer port

Printer set up locally (USB) on an XP PC and shared. Two new Dell Vista PCs
on the network, each with suitable printer driver. When first set up all was
working just fine. 2-3 days later they started to be intermittent.

Recently it is not possible to print from either of the Vista PCs, first
thing in the day for about 1-2 hours. Other XP PCs can print to it just fine.

I looked first thing today, after the PC had been on overnight, with user
logged in but PC locked. I used her password to unlock. Printer properties
showed no port defined at all! I tried adding but "Add port dialog cannot be
displayed. Operation could not be completed (error 0x0000007b)." I tried to
enter properties as (network) administrator: "Printer properties cannot be
displayed. Access is denied"

While still trying to trace the solution the port appeared and printing was
OK. However when I looked at printer properties I could do that as the user
but when I tried to do that as administrator I could not! “Printer properties
cannot be displayed. Access is denied"

It’s driving me nuts. Any ideas, please? There are no scheduled tasks
which suddenly start and I had neither logged off/on nor restarted the PC.
The other Vista PC also started to print at much the same time. The other
two XPs in the department could print all along.