Yes I had instability problems, tried to use it with a valve car radio (Radiomobile) as it did not have a power amp. Never worked properly, made a 1 Watt transistor amp using AC 128s, that worked fine! The Radio was for my 1961 Isetta 300+ which had positive earth.
There is some info here on the IC-12 and other sinclair items.
http://www.nvg.ntnu.no/sinclair/audio/amps.htm
Somewhere in there it does say that the IC-12 seems very similar to a Texas chip!.
Best regards