my 1988 mustang gt convertible usually runs great but sometimes it idles up and down and will even die. the problem stops when i accelerate but will continue at cruising speed. sometimes it does not happen for days, then when it happens it can last for a few seconds up to a day. i have replaced the cap, rotor, plugs, wires, fuel filter and egr valve. it feels like a vacuum issue, but i do not think a vacuum issue would come and go like this. i am lost! please help