runs rough,drives fine until hot, if I stop at a red light, then proceed will start to buck and seems to lose power,when bucking stops drives fine also after stopping for about 15mins. cranks but will not start for about an hour then starts fine but still runs rough. I did a tune up(plugs,cap,rotor,wires,in the process had to replace the rad. Any clue to repair problem will be appriciated truck is a 91' chevy full size blazer 5.7ltr. 4X4, Thank you.