Thanks for the hint re. chopper isolation. Charge injection could be an issue, and I have not thought about that yet.
I had tested the input resistance, it varied, some ball park was 10 meg., it varied, hence the input drift.
The problem I have with stability in principle is acutally similar to the one e61_phil has, at a higher level. I have read somewhere on voltnuts someone had an issue with a protection diode in the input, causing noise/leaks, and mine could be just more leaking, just at a much higher level.
As far as parts are concerend, I think the IFN401 is a good bet, and re. input low leak protection diodes, the PN4117A jfet seems a good solution, so I may try this. Any opinions?
e61-phil: I am in the Munich area. Not sure swapping (at this stage) is the best for both of us, these are rare parts, mine may be broken, while yours at least are only a little noisy. Thanks, I may come back.
Still trying to find parts. The ones I mentioned should be ok, other suggestions wellcome.