Ok, thanks both. I was stupid.

Now, I 've got a wonderful oscillator!

No matter the level of P,I,D (except for 0) it does oscillate.

I suppose it is a matter of tuning, but even with the trimmer as analog inputs to the P,I,D variables, it seems hard to fine tune it.

When I think I done it, after a power cycle it start again at a rate of 2-5Hz.

Ioannis