Loss in gas mileage

My name is Andrew, I drive a 1997 Ford Escort Base, it has 136,000 miles, with a 2.0L 110hp 4cyl 5 speed manual transmission

my problem is kinda complicated. For many years, I was getting an average of 33-35mpg on my car. The head gasket has been replaced twice. Lately, I have been only getting about 21-23mpg and the only fuel it seems to perform that well on is Mobil gas. The car has very slow response time coming from a dead stop, usually about 10 seconds to get up to 30mph. Highway speeds, the car doesn't have any problems.

the car does not leak fuel, and it doesn't show signs of it burning it excessively. The excessive heat makes these problems worse.

The fuel pump is brand new, and the fuel filter has only about 10,000 miles.

HELP!