4l60e is the worst transmission Ive ever had to deal with....period. You can spend thousands of dollars rebuilding, replacing and baby on it all you want. fact is that its a poorly designed transmission that is cheaply built.
4l603 #1- felt slight slippage between 1-2nd shift and 2-3rd shift for about a month...suddenly tranny lost all gears driving across courtney campbell
4l60e #2- 2nd gear completely vanished with no signs of slippage or problem
4l60e #3- (Rebuild of #2) 2-3rd begins slipping and shuttering as you describe....goes on for about 2 weeks...notice after 2nd week that when floored, feels like transmission is not engaged with driveshaft....finally lose 2nd and 3rd gear within another 2 weeks.
4l60e #4- (Junkyard pull/rebuild) So far so good, the main problem is that the 4l60e overheats extremely easy. To be prepared not to go through what I went through....spend the extra money on an aftermarket transmission cooler, Corvette Servo (Less overall stress on pumps) and a shift improver kit.