I have a little '98 Dodge Avenger (Lucy) that I bought after selling Lily, my '97 1/2 ton Dodge Ram to Dad. (I still have visitation rights.) When I first got her she drove fine. After a while, she started doing this little bumping thing while idling or driving very slowly. It almost felt like the engine was missing, but Dad played with it for a while and said it felt like the tranny was always trying to shift.
I don't have the money for tranny problems right now, so I was wandering the car section at Wally-World when I saw some Slick 50 stuff. I grew up using STP in Lily & Nelly, the '84 3/4 ton Ford diesel I used to drive, but I went ahead & picked up a bottle.
After going through 1 tank of that stuff, I thought she felt a little better, so I went back & got another.
She's still not perfectly smooth, but the improvement is remarkable.