I am having a sudden problem with the first gear in my Saturn SC2. From a complete stop, and only when it is in drive, acceleration is very slow until I reach second gear/about 20 MPH. This takes around ten seconds; afterwards the other higher gears function perfectly fine. This does not happen when in reverse. This issue only began today, after about an hour's worth of highway driving. I am still able to reach highway speeds, once I am past the 20 MPH hurdle, and can maintain such speeds seemingly indefinitely. I can really depress the pedal and get very little power from the engine, but once I hit that 20 MPH gear change, I get the full RPMs I am used to. Are there any explanations to why this is happening? I have seen a variety of potential answers, such as a spark plug issue, a clogged catalytic converter, or a greater problem with the transmission, and am hoping for a general idea of what would be the most plausible explanation in my specific situation.