Sounds like a misfire. You will need to scan for codes first and post up. This could be many things so if you don't normally work on your own car then use this link to find a good shop near you.
https://repairpal.com/repair-shops
Alex
RepairPal Certified shops can diagnose and repair your 1999 Mercedes-Benz SLK230. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.