Smart 450 CDI max. 120 kmh in 5th course

Hello together, I have been driving a Smart EZ 02/2002 for about 1000 km. 140 Tkm down. Since I live in Brandenburger Flachland this error can not come from the mountains In the 6th gear the car is absolutely weak and in the 5 it only creates 120 at full speed. The car has a technical history unknown to me. Since the car otherwise starts normal and drives inconspicuously except for the missing power I need your help. Here a few data : oil new air filter new (change had small input Luss on Vmax) AGR already with “decommissioned” consumption between 3.9 and 4.5 liters no detectable oil consumption on the last 1000 km of the engine turns the gears fully off to the 6. Where can I start troubleshooting ? Torxbox is ready and the screw I am also powerful. Greetings from the district PM