Have a 1994 Mustang GT that idles a little rough(tach bounces slightly but intermittently) Under acceleration there is a "popping" noise almost like back pressure in the exhaust as well as while cruising at normal speeds. The tach also bounces when this happens. There is no pattern to it. Totally random sputtering and popping. I thought timing, so purchased a new dizzy cap and rotor but did not help. Seems to be less pronounced at night or in cooler weather and very bad late afternoon. Barely wants to run in the afternoon, but idles fairly normal. Ran injector cleaner through it along with octane boost and that seemed to help a bit. I have heard MANY ideas from the fuel pump to the EGR valve, but nothing definitive and no one posted anything exactly like my problem. Help!