LimaWonder

I have a 2002 c220 cdi with 400k kms and the car feels heavy Like the steering , the clutch but sometimes when i put on gas with aditive or use a diesel treatment the car feels super soft compared to his normal state ,but when i turn it off and back on he goes back to heavy.
I canīt understand what might be the problem and i dont want to change things that wont help my problem , so if anyone could give me advise i would appreciate .
Thanks