For a while now, the car randomly acts up, rpms bounce, car revs up and down then eventually dies. This only happens once a week. It's does all this and dies, then when I start it up it idles with higher rpms and when I let off gas it stays at current rpms for a bit then eventually falls. Sometimes it even runs the rpms up when driving! Mechanic said it's the ETC and can't fix it as it can't only be fixed by ford.. Advice?