Check the hoses going into the box on the left fenderwell -; there are sensors in the box that feed info into the ecu and adjust the carb. If the hoses split or get chewed on by critters, the vacuum leak it causes will cause issues just like what you are having. I had an 88 Dodge Ram 50 sport cab with the 2.6 that did exactly what yours is doing and it was the main hose from the carb to the box had a 1/4 " hole in it and it was inside the box where you couldn't see it.