my fox was running fine up until a week ago. now when i floor it it doesnt pull hard, kind of hiccups. im also starting to hear weird pops when i let off the gas which i think is misfire. the idle is good for the most part but just yesterday i noticed it drop a little 2-3 times over the period of 3-4 minutes. i changed the spark plugs, wires, distributor cap and rotor, and fuel filter but the issue is still the same. what do i do?