Jetta - Volkswagen :: 2004 - Reoccurring Misfire / Dropped Mileage

Recently (5 weeks ago), my 2004 Jetta misfired, which caused the car to shake very noticeably, especially at lower speeds or while idling. The misfire was accompanied by the smell of fuel on the driver side and triggered the check engine light to flash. I took it to the shop (a VW dealer). They ran diagnostics, found it signaled a misfire (but no other major issues/sensors) and told me it was due to the spark plugs/wires. I was dubious, as they were not too old, but I allowed them to replace them anyway, as they assured me this would fix the problem.

Just this week, however, another misfire occurred, also accompanied by the smell of fuel and shaking. This time, the check engine light remained on (constant) for 2 days, and on the third day, it was off. I brought it back to the same shop to try to figure out the real cause of the misfire, which obviously was not the previous spark plugs. My city gas mileage has also decreased over the past month - down to about 20-21 mpg from 23-24. What could be causing such a reoccurring misfire without signaling an issue with a sensor or other part?