hesitation

Hi I have an hesitation on acceloration and on cruising speed only when the outside temp is hot. most of the time I can not bring it passed 50 mph, when I try to accelorate passes 50 mph it stumbles and misses, Like I said , it only happens on very hot days, it will not happen in the winter. Does any one have an idea what I should look for? thanx chev lumina 1995 apv