Why my 1997 expedition shakes on freeway when steps on gas going more than 55mph?

ok so i have check engine light on on street car is good but on freeway when going fast like 60mph and step on has it shakes and check engine starts blinking until car comes outta the freeway it stops. in the middle of the muffler it has a hole like a dime big idk if thats the problem when i was changing the brakes i saw it.. it has tune-up aready changed....sometimes car runs great step on the gas a lil and it flys but sometimes just shakes and doest want to speed up......