It's not because of Leopard and also not because of VW2008!
I'm working in the same Company since 6years and we had this problem the first time we switched to OS X with a HP 750C, the second time we upgraded to VW 11, printing on a HP 1055CM, the third time we upgraded to VW 12. Until now, after some system updates the issues ceased, although it was absolutely unpredictable under which circumstances it still occured. Now my big hope was that the issues would be solved with VW 2008 and Leopard. But this was false hope
I think this is extremely annoying, especially as HP told us: We can't do anything and there won't be any driver updates (the last is from summer 2006) and VW: We know the problem, but don't know what it causes, and why are you using patterns anyway, they're so much out of date....
Maybe it's time to pester Apple Inc. with this problem? Their answer's surely going to be of muuuuuuch help...