SLK 230 engine is not running properly

Hello dear Motor-Talk municipality I bought a Mercedes SLK 230 (BJ 98) with 111000km a few days ago. Today I was on the highway and had to find out that the car has at times little power and only about 180km/h runs but then suddenly runs completely normal again from one moment to the other. It seems to me as if someone would turn the power on and off again at the push of a button if you understand what I mean by it. The vehicle was also about half a year, maybe it has something to do with it? What I still noticed was the verbauch. On a short distance (max 10km) at low temperatures the vehicle used over 100km a good 21 liter. On a long motorway ride with 120km/h only 7 liters. Is this discrepancy normal? I think the consumption on short distances seems quite high. I hope you can help me further. Thanks in advance