86 ranger stall and dies when swifted to reverse

I own a 86 ranger, 2.3l, auto, fi,150k. when i swift from park to reverse, I hear a thump sound, then the engine shakes and dies. I recently flush and change trans oil, new cap, rotor and plugs. Sometimes it also stall and dies from park to drive, but not all the time. please help.

Start with the idle speed. Clean the throttle plate and IAC .

Taillight, thanks for responding. How do I adjust the idle speed, nameplate in front sez idle can't be adjusted, also what is IAC and where is it located? Thanks again for your help.

First check to see if the idle speed is to slow. Then get some carb cleaner and clean the throttle plate by removing the throttle box and cleaning the backside of the plate. The IAC motor (idle aie control) should be on the throttle box. It is the idle air bypass that controls the idle speed.