My vans engine light was on and mis fire when I get to around 30-40 mph on 2002 Ford Windstar

my mechanic installed new wires, plugs engine light came back on annd still missed at that speed he then put it on diagnostic and a smoke test he then installed upper intake gasket set , EGR Control Solenoid I drove it around for about 10 miles it still miss at 30-40mph my mechanic said to always drive with overdrive off in town it will drive smoother and then the engine light came on after he said that WHAT ELSE COULD IT BE ?

Asked by for the 2002 Ford Windstar