Ok, I've got a 1989 F-250 7.3L IDI w/ 5 speed, stock except for a K&N which I just cleaned. I live in Reno, which is all hills. Some days, the truck will make it up hills just fine (on the freeway) and hold speed, or even be able to accelerate. I'm usually doing 65-70mph. Generally I never bother downshifting - it hardly makes a difference and it just makes her scream up the hills (about 3000rpm for a mile or two, sometimes more). The problem comes in where on random days, it just doesn't want to climb the hills with any sort of power, speed will drop to about 50mph, even if I downshift.

Why? Why would it have more power on some days than others? lol Temperature has been pretty even in the 30s-low 40s. Actually, a little while ago it was negative 22F one morning. Ironically enough, she overheated that day...