What caused shutter? When I first experienced this sensation I thought my transmission was going so I took it in to get looked at. I was told that nothing was wrong with it except that the fluid needed changed so I went ahead and had them change it. When I got it back the shutter was gone and the shifts were smoother. Soon after I noticed that Valvoline makes a tranny fluid with shutter protection. But I still don't know what caused shutter.
Also my car starts experiencing shutter about every 6,000 mi. so I have to change the fluid, once I do it goes away. Changing the fluid is not all that bad, I just use a siphon pump and take it up through the dip stick tube. The fact that I have to change it so often makes me think there is something wrong with it.

Mark