Car sits for a few hours it will not start?

I have a 94 camry that starts and runs great when it's warm out, but when it gets below 10 degrees outside and the car sits for a few hours it will not start. It will give one quick pop on the first turn of the engine then just crank over without firing. If I give it a shot of starting fluid it will start but run really rough, with the check engine light on. After a minute or two of keeping the r's up around 2500 it will smooth out and the ck engine light goes out and it runs fine. It will then start fine until it sits for about 3-4 hours in the cold. I plug it in but am not sure if the frost plug heater works or not and since I live in Duluth Mn. It is below 10 degrees about 9 months out of the year. Also I have replaced the plugs wires cap rotor coil air filter all the basic tune up stuff. Any help would be great.

Thanks Rich H.