Well, I am assuming the head was redone by a competent machine shop. I would further assume that if they thought the head needed machining, they would have recommended it. I would think they would actually check it for flatness while it's in the shop. So I find it hard to believe it's anything due to the head itself.
The dropping down to 25 or so on steep hills, I would expect that out of a 5.9 moving a 9-ton bus up a steep grade.
Since I have not done a formal diagnostic on the bus, it's kind of hard for me to say, but *something* is making the oil pressure run high (water/antifreeze in the oil would probably do the opposite). It could be a gummed-up sensor. But blowing something out of the tube ... it could be the air compressor could use a rebuild. How well does it build up and maintain air pressure? Any residue in the tanks? Specifically, any oily or sludge-y residue? (A little water is normal in the primary air tank, as long as it's reasonably clean).