Maybe I misunderstood what you typed but did it sit for an extended period of time between when it last ran good, and now when it isn't running good?
My first guess at your issue is some sort of vacuum leak somewhere but old gas might not be helping. I was chasing ghosts in my car (still am) and some of them just disappeared as I finished off my tank with gas that was at least 4-6 months old and filled it up with fresh fuel.
