I have a 96 F150,132000 km,4.9 six,5 spd.The truck works fine in town,but on the highway at high speed after a few miles it will lose power ,miss and shimmy,sounds like it back fires thruthe throttle body.I can spray cleaner in it and it smooths out,but after a few more miles it acts up again.

Looking for some thoughts on what the trouble could be.