Grand am had service engine light on, I went to start it and it began to sputter and shake. There was a sulfur smell coming from the exhaust pipes. I went to drive it and when I tried to go up hil and couldnt get it over 10 mph. While acellerating on highway I could feel it jumping speeds and the sputtering and shaking happening when I would have to slow down and excelerate again. Also while I was driving the service engine light was blinking. Is this the CAT or O2 sensor, or something completely different any advice would help....thx