Windows 7 will not release ports until printer is uninstalled

Question

I have Windows 7 installed on about 45 PCs, and about twice a week this happens to someone. The symptom people come to me with is that they suddenly cannot print to one of their printers.

If it's the Canon 5035: Nothing is in the print spooler and prints do not show up in it even for a moment. Files do not appear in c:\windows\system32\spool\printers.

If it's the Oce PW 300: Print jobs show up in the spool list, but are stuck there until I manually clear the spooler folder.

Restarting the print spooler, manually clearing the print spooler, and rebooting the PC do not resolve the problem. Uninstalling the printer, rebooting, and reinstalling the printer does not work, either. I have to uninstall the printer, delete the port, reboot, then reinstall the printer and the port. One PC needed the spooler service stopped before I could delete the port. So far, this only happens once to each user (I'd say it's happened to about 15 so far) over the past six weeks or so.

Six weeks ago is when we expanded to a second suite in the same building and duplicated our setup. We have a Canon IR Advanced 5035 and Oce Plotwave 300 in both suites. We use direct IP print as we have not yet gotten approval to join this office to the domain (long story). It occurs on the Canons at a ratio of 2 to 1. Not all of the affected users have the printers from both suites installed. All of these users have a virtual desktop to the customer that they use, but I can't identify a pattern related to that. The PCs are a mixture of Lenovo T520's, HP z400's, and 8560w's.

How do I identify what is causing this, and is there a change I can make to the printers that will prevent this from eventually happening to the rest of the users?