Best guess - Fuel Pump Relay.
They often fail on 190s and they can show various symptoms. Mine started by
dying just after start-up, progressed to doing what your friend's is doing,
then when the engine quit at 50 mph I replaced the relay. That was three
years ago, and no worries since.
1986 190E 2.3-16
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.