That is an interesting problem.
It sounds like the security of your machine is effecting the local print spooler service. Id assume that some security action is failing or timing out while in the Admin group that is normally skipped for normal users. Unfortunetly I do not have any suggestions on what that could be.
For a situation like this you may want to consider downloading and installing ProcMon from Microsoft. Running ProcMon while printing a document as Admin may help you isolate what processes are running in the background and timing out. Perhaps if you can pinpoint the specific process that is timing out you can find a solution for it.
Otherwise you mentioned trying the UPD 5.7 PCL6 driver. Try swapping over to the UPD PS or PCL5 driver and see if there are any consistencies. It would also be interesting to see if this same problem is duplicated over a normal USB installation.
Cant tell right now if this is a local printing problem on the machine or some kind of a network security conflict. I suggest working on the local printing problem first and then expanding into the network configuration area later.