2003 engine reving.rpm/ sputtering problems

Hi I recently started having a really strange problem well driving to work. It seemed to me like the car's engine was working at a way higher RPM then normal when I was going 110KM to work. THen I wasnt able to rev the engine past 4400pm well idle. It's been consistently -15 for the last like 3 weeks, and wherever i'm driving it seems like the RPM's are way higher for any speed then it normally is, and I got around 150 less km to my tank off gas then Normal. When I rev the engine in park at 4400pm it seems like the engine is misfiring and I can hear sputters coming out of the exhuast. I had the spark plugs replaced last year too. ANy idea's? I thought it might be the fuel quality or the injectors but i'm not to sure.

How many km`s on car ? Have you changed the plug wires as well as fuel and air filter? Is there a SES light on indicating a problem . it could be fuel related problem. A very reputable Gm parts guy told me not long ago to run Petro Can or Shell fuel in car as they do put additives in their fuel to help clean fuel system, you pay more for it but he claims it helps.