I have an 87 S10 2.8L V6. It had been running perfectly. Then last weekend we started getting very humid weather and we got over 2 inches of rain. Then weather has remained very humid. It has started “chugging” when it kicks into overdrive, and when I come to a stop it idles way down to where the Oil pressure just about bottoms out and idles roughly. Would it be the oxygen sensor or the plug wire/distributor?
More likely to be the ignition wires and/or distributor cap. Replace those first, along with the rotor in the distributor. If the oxygen sensor has never been changed, it’s probably due, but that’s not likely to be the main problem here.