Recently my 308 started running rough. At first it was only during warm-up. She'd idle slow and backfire as I backed it out of garrage. If I gently added gas, she'd jump from the slow idle of 500 RPM up to the normal 1100. If I ran it slowly, it would run at about half power and then suddenly kick into faster mode. I think it migh tbe a fouled injector, bad spark generator, what? Now, after about two weeks of casual driving it's always idling at 500, and is sluggish at the low revs. What's wrong with my baby and how can I fix her up?