car starts to "sputter" a 55-65. Is it fuel or emissions? engine light on

What is your question?

Car runs well after I added super octane fuel.. but I added High octane fuel for a long trip and after 175 miles and a FILL UP (93 octane- supposedly) suddenly started "stuttering" again as I'd get toward 60- 70. Stopped and purchased STP fuel cleaner/additive..(the good stuff) NS IT GOT BETTER..but engine light did not go off. Still sputter a bit. Is it FUEL or is it emissions?