This morning after about 5 minutes of driving my truck started to stumble badly. During normal driving (coasting with the foot steady) while going up a incline, it will stumble like it's missing. At WOT, it seems to be fine. It also has a bad idle that comes and goes every few seconds, even in PARK. I scanned the computer and came up with a code: P0442 Evaporative Emmission Control System Leak Detected (Small Leak). I swapped gas caps but that didn't work. Any ideas? Last night, I did fill up the tank with gas, so could I have bad gas?