Cold weather miss at low rpms

[D_French]

Hi. I'm Davis and I'm new here. Here's my problem; 89' 4 cyl. runs fine all summer, in the winter though... It starts up fine and runs well for about twenty minutes at highway speeds (75) then the "check engine" light will flicker, maybe stay on 5-10 seconds then go out sporadically. When I get off the freeway and drive around town in 2nd or 3rd gear it coughs and sputters especially while decelerating. If I push the clutch it idles just fine, but under load coughs and jerks (no "check engine"). At my wife's suggestion I'm changing the fuel filter and checking the plugs, I have also put "HEET" in the tank. A friend of mine said it was the o2 sensor, but the previous owner removed the cat and sensor so that's not it. Any clues?