We have been experiencing this same issue with our HP Laserjet 600 M602 printers.
Was able to resolve by setting the Print Mode for all papertypes used to the same setting. Orginally we thought it may be a tray setting issue or firmware problem but in reality was the paper type settings. We setup printer for plain paper in tray 2 and bond in tray 3, by default plain was set to Light print mode and Bond to Bond print mode. Every time printer switched trays it pauses to configure the different paper type settings.
Hope this helps others resolve their printing issues
Enjoy Your Day,