After I changed the ignition switch I was sure to fixed the problem. But damn, the engine still dies during driving. Favored places are crossroads. So, I took the next step and changed the fuel pump relais. Part number in the 924S catalog is 993-615-227-00. The relais has easy plug in contacts. For better handling uncrew the upper fuse-holder to change the relais. Since I changed the relais 3 days ago the engine runs without the "dead on crossroad" issue.
If you still have the owner's manual it is easy to locate the relais.
It is position II on the picture
In reality it looks like this under your dashboard
new relais
old relais