Mechanic said that all cylinders are firering.
No engine service light comes on.

All of your ideas are great. It may be the O2 sensor. It could be the fuel filter too. Maybe one of the 12 spark plugs aren't firering properly. Let me add, the symptoms are hard to notice while driving, but you can sense them. The hesitation is irregular; it’s not predictable or consistent.

What do you think of the warrenty?