1994 Aerostar, 120,000 miles

My Aerostar started running rough two weeks ago. Then it ran great again for a few days. Now it might run fine for a few miles, then rough for a while. I changed the plugs, wires, dist cap, rotor last year at 110,000. It has always run great. A mechanic told me to put some injector cleaning in the gas. I did and it is still having problems.

Thanks.