ECOTEC engine?
If so it sounds like the crank case breathers are becoming choke with oil and dust. Quick fix is to replace these breather hoses. If you get Vauxhall to do it they have an additional mod that is supposed to prevent the oil rising into the hoses, but in true Vauxhall tradition, it isn't cheap.
I will attempt to find a link that explains this better.
Mike
January 2005