That has been a fairly common issue that I have seen. The key is that as the temperatures inside increase you get normal shifting. Now I'm not a big advocate of additives but the only other option you have would be a tear down and reseal.
If you check engine light is still on, head over to a parts store like AutoZone and have the code read. Maybe that will add something to what is going on.
David here's what happened upon following your insite. Went to advanced auto and check codes. It said I needed a vss (vehicle speed sensor). Bought the part for $48. After much frustration with the little bolt to remove the old, used sawzall to cut the part out. Installed new part and took a ride. Check engine lite was off. (good). Went down the road to the DQ ready tocekebrate. It shift good one time. Then the problem returned with not shifting at all but the check engine light stayed off.
On the way home however, it began to shift correctly and symultaneously the check engine light came back on. Ate the ice cream and went to bed. Called a recommended auto trans place and they said I could come in and reread the codes and they would let me know what it would cost. They fund another code came up the the output sensor was faulty. replaced and charged me $80. Problem solved. Thanks