My 1989 Saab 900 turbo 16v seems to buck when i lightly press the gas or just randomly when im lightly on the gas to maintain speed. Whenever these bucks happen the rpm needle goes from 0 to about 4k back and forth. I have replaced the alternator, fuel filter, o2 sensor, checked vacuum lines and cleaned the AMM. Spark plugs where changed about 3 months ago. Sometimes when i lightly press the gas the rpms go to 0 and the CEL comes on for a few seconds until the rpms go back to normal. I also have a small crack in my muffler but i dont think that would cause all of this.