3110cn PS Disappearing Envelope Jobs

When we try to print envelopes from Word 2010 to the PostScript emulator the job doesn't appear at the printer.

The job goes through the spooler, or is passed directly via IP, but nothing happens at the printer. We would expect a "load MPF" or something similar.

Envelope printing used to work from Office XP via the same drivers. I think (hard to rememeber) it also work from 2010 a few times but it was never stable. If I pick aother PC at random on occasion envelops jobs work but I can't see a difference in the PCs - same drivers, word version, procedure....

If I send to the HP emulator from a proberm PC I get a data error PDL Request 816-720 and instructions to cancel the job.

The same PC/Word session can print to an HP printer with MPF.

I uninstalled and re-installed the drivers on the PC.

Ideas?