1989 740Gle b234f 16 valve

While highway driving, ingition will cutout as if key is turned off and on. The tach drops sharply then resumes, sometimes completely to 0 rpm and right back up. Seems to happen only after car is hot - usually 15 to 20 minutes into trip. It doesn't happen all the time, only some days. Nothing else stops or flashes or dims, just ignition firing fails, as if engine dies and then restarts. Sometimes this will happen once or twice back to back, somtimes it will continue for a couple of minutes then stop as mysteriously as it stated and the car runs fine. Sometimes it starts acting up when I am in city driving, it will idle fine at a stop and dies when I begin to load engine. It will usually start right back up. Starts acting up in city driving usually if I have been at idle for several minutes, like at a drive thru window.

I replaced the ingiton switch but it didnt fix the problem. Just a bad guess as to my problem :-(. Any help on this would be much appreciated.