Take a compression test of all the cylinders, and see what they read, opposed to what they should.
Dirty Fuel filter perhaps?They use 85 usually in high elevation states like Colorado where the air is thinner. It's the same as using 87 here, or elsewhere. Imo, if your engine was designed to run 87, thats what you use. A higher octane will not make it run any better unless you have an engine designed with variable valve timing, and variable electronic ignition timing, and even then it might not be all that noticeable.
Sounds more to me like your engine is tired, which is very common for that mileage .
It's the altitude.could it be the 85 octane fuel they have out west that contributed to struggling in 5th gear up the elevations
along with the thinner atmosphere
i tried the most massive grades that are local in Iowa and Missouri this weekend and did not have the same issues of puttering out in 5th gear with 87 octane and much less elevation