Senior Bible nailed it...as usual. Something's awry; have it checked out.
On tuneup notes, ensure that your monobelt tensioner's up to spec. If it's never been changed it's likely due. An ounce of prevention, as they say.
I also agree on the plug wires. Replace them when/if you break one.
Lastly, I'd replace the load-leveling filter...it's in the bottom of the reservoir (if your car's so equipped, which it likely isn't but I thought I'd mention for the consideration of my fellow W124 300TE/500E owners)
All the best, Michael