Thanks for the answers. It is an Rider 11 Bioclip, no idea about the age/hours but it certainly is an older machine. I guess that it is at least 15 years old.
When I was searching for the coil I noticed a wire were the insulation was was rubbed of. I put some heat shrink around it and tested the engine again. It now runs much better and doesn't stall anymore. But it still stutters a bit.
We put fresh gas in it about two weeks ago, before that the tank was empty. After that we did mow the lawn 2 times without any problems, the 3rd time the problems began.
@slomo, no I don't check the oil every time I mow the lawn. Seen the issues we now face I assume cleaning the carburetor is still a good thing to do?