Check the throttle and IACV, physically. It scould be something sticking, like the IACV. it can get dirt and shit in it over the years, its not hard to take off, and can be cleaned sometimes with a spray called "deep creep" which is like a draino chemical for cars, it cleans out stuff by dissolving crud and oil and that. also carb cleaner can be sprayed into the throttle body which could also get stuff on it and stick. this can happen on cars as old as ours just by exposure to the road and dirt and dust and all. nothing big, but cleaning this sort of stuff every few years helps, though if you always keep a clean air filter, it may be in better shape, though the recirculation can add oil into it over time.