PCL6 vs PCL5e driver problems on HP Laserjets

We have HP Laserjet 4350 printers and are having printer driver issues. As we were frequently getting 'PCL XL Error' messages using the PCL6 drivers, we changed to PCL5e drivers. The PCL5e drivers won't print correctly from one of our programmes, Pertax. I have spoken to the Pertax people and they say their programme has no printer commands in it at all, the printing is controlled completely by the printer, therefore they can't help.
With the PCL6 driver, Pertax would only print to a particular tray if you specify the paper type. There are 4 trays on each printer: tray 4 = plain, tray 3 = bond, tray 2 = letterhead: if you select 'tray 2' it just prints to the 'default tray' which is tray 4, but when you select 'letterhead' it prints to the correct tray.
With the PCL5e driver, no matter what you select it prints to tray 4.
Can anyone suggest how we can sort this out? Thanks.