In cold weather truck sputters a lot and emits a lot of smoke but once it warms up the smoke stops

I have an 1989 Ford Ranger, 2.3L 5-Speed transmission. Been having a problem with the truck during really cold weather, lately it's been in the 20's. First off it takes a little while to warm up the truck enough to move it, probably 5-10 minutes before I can safely pull out on the road. It's sputters a lot and emits a lot of smoke, and once it warms up the smoke stops. The other problem is when i'm on the highway, I can get the truck up to 75 mph, but that's in fourth gear, with the engine almost screaming at me, but once I put it in fifth gear, I can barely keep the truck maintaining speed, and if I go up a slight incline in the road, I will slow down, with my foot on the floor. I have to shift back to fourth just to keep up with traffic. Strange thing is this, it's now in the upper 40's this week and the truck is running much better, no sputtering and smoking like when it's really cold out, and 5th gear at least maintains speed and even ever so slightly speeds up the truck. Please give me some suggestions for this problem, I've already changed the spark plugs, fuel filter, and fuel pump (old one was shooting fuel through a rotted hole in one of the lines), going to change the wires this weekend, and I have tested the ignition coils with an ohm meter and they test good. Thank you in advance for any help at all.