I'm expecting a call from Bell today, so will post the reply. The first guy I spoke to over there suggested the change in pressure with altitude.
Most definitely let us know what Bell tells you. I'm going to stick with my original theory. Something isn't right. A slight drop in pressure I could live with, but from mid-green to just above the redline suggests you have a boost pump problem somewhere. JHR might be on to something. His post reminded me, I've had a stuck valve before, and it did cause one pump to recirculate. Also, if you do have a slight air leak, it won't cause a flame out, but it will cause the offending pump to show a pressure drop.
Any chance you could borrow a boost pump from another operator to use on your aircraft to see if you can isolate a problem?