My inline 300 six runs great at 75°+ outdoor temperature, but when temperature drops or humidity goes up it runs like crap. I replaced spark plugs, wires, cap and rotor no change.

If I was a betting man, I'd wager your fuel pump is getting weak, and those temp fluctuations, which can affect the performance of the entire EVAP system, are causing a loss of fuel pressure at the lower ambient temps or high humidity levels. To start with, since you've already done most of a tune up, you should clean the MAF sensor, PCV valve and hose/elbow, EGR, and triple check for vacuum leaks. Then check the performance of the EVAP solenoids, and clean the charcoal filter. It would be nice if you can hook up a fuel pressure gauge and mount it to where you can read it while driving, I bet you would see it drop or fluctuate when those conditions you describe happen.