1982 Audi 4000s, 1.7L, Manual transmission, 79,000mi

My 82' Audi is experiencing some very reduced engine power. It feels like I am towing something. The engine doesn't seem to be hesitating or sputtering at all, it just feels like it isn't all there. Recently, my OXS warning light came on and was informed that it was no big deal and the manual says that it just comes on every 30,000 miles as a reminder for a maintenance check. If that was so, it would come on at 90,000 miles, not 79,000. The OXS light came on around the time the car lost it's get-up&go-go. I think the car to be running rich because of the fact that it's sluggish and it's not getting optimal gas milage. Also, I changed spark plugs and wires and noticed that a couple of the plugs had pretty nasty oxidized look and a couple were fouled or pretty wet. With the new plugs and wires the car seems a bit happier, but that isn't the entire problem. Any feedback is welcome and greatly appreciated.