I had this exact same problem on my 89. I checked the brake booster cause it only happened when I stepped on the brake pedal. The problem ended up being the idle air control valve was carboned up causing the ball not to seat right. It's part of the throttle body. I didn't take pic's but I took the thottle body off, removed the 4 screws. There is a cap that has 4 notches & a hole in the center where the ball seats. I used a screw driver to remove the cap. Count how many turns it takes to remove so you can put it back in the same spot when you put it back together. Remove & clean everything, even the coolant passages. A new throttle body is around $400 so I would try this first
