Hmm, it's annoying, isn't it? They do seem to have a thermostat - they get hot, go out, then come back on when the seat gets cool. Not very good in my humble opinion, definitely caused by a thermostat, methinks.
I've only had my V reg IS200 a few weeks, I am well pleased with it apart from the climate control which doesn't do the best job of regulating the temperature (either too hot or too cold), and the seat heaters.
I had a Golf GTi before, the seat heaters kept a constant temperature, which could be adjusted. And the climate control did not have to keep being adjusted to maintain a comfy temperature.
If I can only ever think of these two things to moan about that's not too bad, apart from the the cars a good 'un.
I was wondering if mine had a dicky thermostat, but now I'm not so sure, I think it's bad thermostat design.
Have fun..
Paul