I have built the circuit with the LF356, including the offset adjustment, and everything worked perfectly while testing with a 9V battery. But in 'real world' I swapped by mistake the input/output cables and fried the chip. Had another one as backup, NJM4580, and it is doing its job as expected.

One problem solved, but the close loop servo is still "losing steps" That's another story..

Thank you all for your help, much appreciated!