Car is a 2000 Alero 2.4, Automatic, 106,000 miles.
The car had been running sluggish with poor gas mileage.
Last weekend this was done: replaced the spark plugs (worn severely), replaced transmission filter and fluid (not flushed), replaced fuel filter, and replaced battery.
The car ran fine for the first two days of driving.
I then ran 1/3 can of Seafoam through the intake and replaced the air filter.
Now, on a 5 hour road trip (mostly highway), at steady speeds the car would ocasionally "miss" 4 or 5 times, then smooth out. At a stop, in gear, the engine idles smooth, but then occasionally "hiccups", similar to the "miss" at cruise speed. Now, the engine sometimes "stumbles" on acceleration.
There are no codes set or pending, and the SES light has not illuminated. I've read posts on here about the tranny, MAF, or O2 sensors. I'm leaning toward a contaminated O2 sensor because I ran the Seafoam through it.
I'm open to comments and suggestions. Thanks, Bob