engine started missing

2005 SR5 4 wheel drive quad cab with 6 cyl engine. 187,450 miles

Fueled up in a location across the state, but well known station. About 45 miles later had a miss while at interstate speed. Occurred once again, but that was it. Next day, once more. Didn't drive for 2 days. Drove 30 miles on the 3rd day and then about 15 miles into the return trip, significant missing series as I accelerated up from around 45 to 60. Happened again numerous times from there home. Started up this morning and by the time I had driven 3 miles it was almost continuous. Check engine light began flashing. Added fuel additive which treats, cleans injectors, removes moisture, etc. Stuff called Sea Foam Gas Treatment 16oz. Filled gas tank. Would miss even when idling. However, about 10 miles into this treatment, the missing became much less noticeable except when accelerating up steep hill or significant acceleration. Seems to be getting better, but still missing. Can't be the fuel filter since they can't be changed. Check engine light would flash at the worst of the missing and then go solid when it was just purring along. However, this same light comes on when I get ethanol gasoline, which I did at the first location. It takes about 1/2 tank of good gas before it goes back off.

Ideas?

 

Welcome to TW from TX.

x2 on the higher octane.

 

Usually don't have to run ethanol. Most places around here are getting away from it. However, this time it was a necessity. Hopefully the fuel treatment does it.