1992 Ford F150 Wont stay idling - like running on 6 of 8 cy

I replaced the following in 9/2009 - Plugs, plug wires, cap and rotor for routine maintenance. The truck ran like a top as it always has. Never a problem. Truck has sat in outside temps ranging from mid 50's to low 30's for 2 weeks, Stasrted up the truck today and is sounda like it is running on 6 of 8 cylinders and will not stay idle. Can condensation in the fuel tank cause this? What else should I be looking for. I have already reinspected all plugs, wires, etc.

Confused