Well, the amplifier works.
In the end it was a remark by kchriste which lead me to check my power supply (I don't own enough meters to check every point of interest so the behaviour of my power supply was not monitored).
Its strange drop in voltage showed weird things in the amp which I totally misinterpreted.
And also the highlighted, reiterated advice by audioguru may have helped. I tried the amp before it was altogether assembled, assuming that for a last test of a few seconds to minutes the T3 transitor could very well NOT be in thermal contact with the finals. After all the amp would not have time to get warm, no load and all, to make this thermal connection necessary.
And I still don't know what the answer to this is, but now it is all assembled and for the last few hours it works stably and with problems.
Which is nice, now I can build the other 5 amp to complete my electronic crossover-multiamp system.
Thank you
Uwe