My summer toy (which I dont drive often) began to get a little converter shutter as soon as 4th gear lockup occured (40 MPH) though 55MPH. Transmission is a Built 4L60E, Z pack Clutches, Beast Sunshell and alll the other goodies. Converter is an Edge 9.5in 2600 stall lockup. Transmission is 3 years old and only has about 1000 miles on it. I called the place where the transmission was built and he said when it sits for so long , which it has, sometimes it gets dry and the fluid gets bad sittin gthat long. He said give it a drain and refill with Dexron VI and it should be good. Went to Advanced and picked up 8quarts of Castrol VI and low and behold no more shutter and it shifts harder it seems. I'm a happy man!