On my sons 1999 TJ 4.0 3 speed auto, once warmed up the Jeep has a high idle symptom while shifting into reverse. It seems like it shutters but might be just from the idle. No clue where to start. It did stall today while shifting to reverse. Fluid is full and color seems how it should with no burnt smell. Did some searching but people explain trans issues in various ways and didn’t find much other than a trans cooler sticky. Is the trans cooler a typical fix for these symptoms? Is it worth getting it either way?