Hi Thomas, I have the same problem. I do have a double unit and do want to combine them for double power. The right amplifier is doing well, but the left one is oszillating. Did you find a solution yet?
No, no solution until now.
I disabled the onboard bias voltage and fed it from external. So I'm able to slowly increase it from 0V on. But at some point around 1,7V the circuit begins to oscillate.
Another try was to make the input impedance better through cutting the trace after the input isolator and soldering a small coax cable with 50Ohms in there. Circuit still oscillates.
Used one FET or two FETs at the same time. Circuit still oscillates.
I'm really frustrated. Both modules I'm having here are oscillating. Not only one ist stable.
Next try is to clean the downside of the pcb and the heat sink for giving a better ground potential. Maybe there is some oil from the milling machine left there. I've done that before but now I'll do it again very exactly.
Did you had a look at the other thread (https://forum.amsat-dl.org/index.php?thread/51-what-tx-amplifier-do-you-plan-to-use/) here? It seems, some guys there are having also the same problem as we are having.