Hello, sometimes shutting the motor off and restarting it will make the computer reset everything and anything that may have a glitch in it. There have also been some problems with AC actuators that this will cause to clear up temporarily. The only thing I can come up with that could cause your problem is the Pressure Control Solenoid Valve. This is a component that can cause many different types of trans problems. It has been so prevalent that GM has issued a technical bulletin for the technicians, so they would be aware of it. Replacement requires removal of the trans oil pan. This valve controls the trans line pressure of the fluid and that causes problems when it is not correct. Too much makes harsh shifts, to little let them slip into gear or too soft a shift. The correct way to test it is with a hand held class 2 scan tool like the Tech 2 that the GM dealers use and a line pressure gage. This tool will read the codes, which the valve may or may not set. If it does the service soon light will come on. But the nice thing is that with the scan tool the technician can run function tests on the components inside the trans to see if they are functioning correctly or not. Testing usually costs about an hours labor. Below is some info on the valve from the service manual. I wish I could post the bulletin for you, but the file is so large that the site won't accept it for posting, But the GM technical bulletin number is XXXXX Thanks
Pressure Control Solenoid - Valve Asm.