Jim, David: thanks for the hints. Yes, it will be a good idea be to shield the stage. I didn't suspect of external sources because the xtal osc. gave perfect stability, but who knows. I was pointing to weird causes such as surface leak currents, e. g. the FET case had some years' dirt on the bottom.
Light?. Yes, I once used a power RF transistor for a VCO because they recommend operating the active device at low current density to get lower phase noise, but mains frequency sidebands were horrible... till I realized the fluorescent light was penetrating the transistor's ceramic body!.
Listening to the demodulated oscillator output, or amplifying the drain current variations seems also a great idea, maybe it's not popcorn what I hear..
Fortunately I won't need Z5Us at 40MHz, I've also had headaches from their microphonics in the past.
I'll let you know!