I noticed that the dekatron that is counting the tens of minutes sometimes seems to miss a step. It seems to be geting worse when the clock is left on for a longer period of time (several hours). I guess I will have to look into that. I remember that this particular tube has shown some problems before, when I was building. Because it appeared to have a problem in a certain step, I decided back then to have it run in the opposite direction, e.g. counterclockwise in stead of clockwise. Which is kind of strange for a clock, admittedly. But it is as easy as swapping the G1 and G2 connections. 

I have already tried to change the triode that is driving it, but that doesn't solve the problem. So the problem is either in the resistors and capacitors generating the phase shift G1/G2, or it is in the GS12C itself. I still have one or two of these lying around, but still it is a lot of work, because this particular tube does not sit in a socket, but the wires are soldered straight on. Maybe it's best to start with some measurements of the G1/G2 waveforms and then some tweaking.