2003 Transmission Problem

Hi, I bought a used 2003 Avalnche and am having tranny problems. Sometimes it shifts through the gears just fine and other times it gets stuck and won't shift higher than second gear. I have changed the fluid twice and dropped the pan to inspect everything and it all looks shiny new and no metal filings. I did have a leak and put some stop leak in it and then it sat for 3 months. I thought that may be the problem, hence the 2 transmission fluid changes. Any thoughts?

May want to try changing the 1-2 and 2-3 solenoids.  They are what transforms the signals out of the computer to what happens in the valve body.  They've got small orifices that flow fluid based upon whether the computer signals them to be on or off.  Those orifices may be clogging, or the small valves in the solenoids may be sticking.  If you can do it yourself (it isn't too hard if you're comfortable dropping the pan) it should be less than $100 to change those.