I'd run a couple of cans of good injector cleaner thru it. Something like SeaFoam. If you are getting good spark and the air filter is clean and the fuel filter is new, I'd suspect a dirty injector. You need a richer fuel mix for power than you do for cruise. Normally, this means 12-13:1 when in heavy power mode (uphill pulls) vs. 14-16:1 at cruise on level ground. Higher speed also require more fuel. Since the injector only controls fuel quantity by varying the time of injection, a partially clogged injector will tend to lean out the engine enough to misfire, even with the computer trying to correct as it reads the lean condition from the ohtwo sensor. The ecu can't correct enough to overcome the effect of the clog, so the computer sets an error code and you see the cel. The light is intermittent because the problem "cures" itself when you aren't in a heavy pull because the computer resets the code after it can correct for a while.