your exhaust is messed up
i had this happen in my 92 Si. Everyone in a while it would severely cut my power and one day while it was doing that it just blew. It made the loudest sound ive ever heard. Then the exhaust was really loud and i got my power back. I looked back and the muffler hadnt fallen off. when i got home i looked and saw a hole about the size of a penny in the exhaust. Since I was broke I sounded like a rice boy till i got money for a catback. What year is your car?

26 mpg is ok. I get 28 - 32 mpg depending on how i drive. 75mph on the highway usually puts me up there in gas mileage.