Non-stop '88 pulsar nx se problems..HELP!!

I bought a 1988 Nissan Pulsar 1.8L this summer and it ran good for a few months until winter hit. It first started cutting out and stalling so i brought it into a mechanic and found out it was my crank angle sensor, which i replaced. After the replacement, the car ran but barely. If i don't let the car idle for 15-20 minutes before i drive it, it has problems accelerating. Even after it warms up the acceleration is delayed, and it constantly bucks and kicks. This mostly happens when im going 60 km/h in 4th gear at about 3000 rpm's. When I downshift and let up off the gas, i can hear my car backfiring and my gas mileage has been cut down by almost a half. So far ive replaced all of the coils, i put in new spark plugs, and a new oxygen sensor. None of this has seemed to help. I am running out of ideas and i could sure use some help. If anyone has any suggestions it would be greatly appreciated.
Thanks!

hallo... sorry ...i not a full mechanic yet..
but 1 thing u must check on this situation ...the cooling system..

first , it happen on winter ..
second did u fill the cooling system with anti freeze cooling liquid??

when engin not hot enough it will difficure to it to accelation or maybe start up

even the thermostat of the engine see it run well or not?