Honda check engine light

My honda civic hatchback 1991 check engine light comes on and the car starts when I downshift to a lower gear or go really fast however it doesn't do this every time only when I go to fast if the check engine light comes on and I turn the car off the check engine light goes off and it acts as if its back to normal as soon as I accelerate to fast again or rmp it at 5 in first gear check engine light comes back on what might be the problem?