1991 Cadillac Deville Loss of power

Two owner car. On cold starts it runs rough and seems to smooth out when warm. Loses power on the freeway and mileage has dropped in half. Hard to keep it above 58 mph on a hill where I used to run whatever speed I wanted. Some surging and some 'slow downs' on the freeway. No engine noises at all, great maintenance history and a recent tuneup with one injector replaced. No oil loss but terrible gas mileage. Injectors? Valves? Thanks, Terry