All of a sudden my 1990 420sel Mercedes Benz started losing power trying to climb a hill.

I bought a car in NJ on eBay and drove it home to Bransn Mo. The drive home was coming to an uneventful end of the 1,200 mile trip and was twenty miles from home. (Branson Missouri--RT 65 lots of long steep hills) when all of a sudden I noticed the car was slowing down as I was climbing one of the hills. In fact, by the time I reached the top I was down to 25 miles per hour. Even though the engine seemed ok, it did not have an pulling power. It continued to do this for the last 20 miles. I finally figured out to go fast down the hill to gain enough speed to get me over the next hill. When I cam to flat ground, the car still did not have any power, but it did pull itself home. I kept having to feather the gas to keep the car moving forward. Up until this incident I had not had any of problems and for 1,200 miles the car drove great.