Do you know what could be wrong with my car?

I already know I need the head gasket replaced, but I don’t know if this new problem that came up is related to it: Basically the car lurches forward, the ‘check engine’ light comes on, then the car sounds very muffled during acceleration, as if it’s under water. And the really bad part is that the gas goes down a lot more once this all happens, as if its using more gas or something. It’s an old car (2000) and has 140k miles. Thank you.