I have no experience with high altitudes, but that may have something to do with it....It is so hard to tell, and without you knowing what it should feel like, it is hard to tell. Mine felt kind of slow too when I bought it. Then I changed oil, tranny pan drop and filter change, new copper plugs, new wires, new SC oil, and the FWI....it seeemed to wake it up after all that a little bit.