It's a problem with the 4L60E trans in my GMC as well. It only happens when it gets hot, so I try to keep it from getting hot. What happens is the valve that controls the torque converter lockup wears out the valve body, which lets fluid past so it leaks down. Then the torque converter lockup lets go, and the trans "brain" has to lock it up again. It will only do this a few times until it decides there is a problem and puts the trans in panic mode, with full line pressure at all times. That 1-2 shift is the only real hard one, but the other shifts are still a lot firmer than before. Shutting it off for a minute resets everything, so it goes back to normal. The only real "cure" is to fix the valve body, but a trans cooler will help.
