2001 Saturn L200 when hot outside, car slams into gears wit

This happened last year during the heat of the summer and no mechanic could find a problem with my car. When it get shot outside and I am driving with the air on (typically after IU have driven 5-15 miles) my car will rev up higher between gears and then "slam" into the next gear-it jars the entire car. I tried turning off the air and turning on the heat, as well as rolling down the windows- and it seems to lessen the severity. The thermostat never quite reaches the halfway mark and the engine does not steam or anything upon cutoff, so I do not think tha tit is overheating. It just did it Friday (temp of 86) and I am thinking "here we go again". Please give me some suggestions. Thank you so much.