Since it works right at lower altitude, and since the mileage is in line, one can only assume that the IP is working as it should, or at least close enough to be servicable. The problem is not fuel, it's air. Fixing the fuel won't fix the air. The engine still has all the oxygen it needs to burn, but the compression is effectively reduced as your altitude increases. The "fix" is not fuel, but air. I understand your desire not to get into turbo retrofitting, and with that in mind I'd have to add another vote to having a diesel shop (unless you have the equipment) set the timing ahead just a bit. This is the closest you'll get to "emulating" more compression in the engine, which, given only the information in the post, I would have to believe is the root cause of your issues. Of course for the hard starts (not so much the cruising issue), do not neglect the basics, good batteries, good starter, good starter wiring and battery/engine/starter connections to deliver adequate cranking RPM, the glow plug system in good order making sure that every one is good condition and working properly, as every little bit helps during startup. By the very nature of a diesel engine, you are going to notice a difference in performance at different altitudes. Unless you're talking about extreme altitudes, it shouldn't be mind blowing, but the difference will always be there.
