Chugging, sputtering, and stalling

I have a 1987 GMC Sierra Classic 1500. It has about 135,000 miles on it. I bought this truck this past spring and it has always run great. Now that it has been getting colder, it has been chugging, sputering and stalling out on me. I thougth that it may be the fuel filter, but we have had a few 80 degree days the last few days (compared to the 50s and 60s we have been having) and the truck has been running great again. So I have ruled out the fuel filter. This is the first year that the Sierra trucks had fuel injection, other wise it acts like when you have a bad or dirty carborator.

The dew is collecting on the spark plug wires. They may even be original! Change them, distributor cap and rotor, and see the changes.