My wife's 1993 740 wagon is acting up - it doesn't like the cold.

A few weeks ago, we had a super-cold day and her car was very hard to start. After starting, it ran very badly/rough, as if less than all 4 cylinders firing.

The plugs were heavily fouled and it needed a tune-up anyway, so we did all that. New plugs and wires, distributor cap, etc.

The car ran great for a few weeks, until we had another super-cold day, and the same thing happened again. Hard to start, running very rough, fouled plugs.

It seems to do fine as long as the temperature is in the teens or twenties, but if it gets down near zero, it kicks off this problem.

So, my question: What would make this car foul its plugs so badly, but only on very cold days?

Mechanical Level Alert: I have worked on 60's and 70's era cars quite a bit, but I get a little hazy with computers and fuel injection and so on. Use one-syllable words and speak slowly, please ;)