In hot weather tachometer jumps around

My car in hot weather at speeds over 55 mph and approximately 2,500 rpm's showing on the tachometer will drop and then raise back up about 200 rpm's on the tachometer. The car jerks a bit when doing this. I can't tell if the transmission is hunting in and out of high gear or slipping. It does this mainly when the cruise control is engaged but will do it when it's not. The cooler out it is the less frequent the issue. In cold temperatures it rarely does it. There are no error code indications with the lights for the gear selector on the dash screen. I have not checked it with a code reader.