Mosfet regulated 400V PSU oscilation help

I building my second Mosfet regulated PSU. It's supposed to put out around 400V for use in a tube power amp. The first one was an amplified zener. Worked fine, but I wanted to build something more advanced. The attached picture is the current design I'm testing. It works fine unloaded, but starts to oscilate heavily when loaded, to the point the output voltage drops to only 160V. Depending on varying component and load values, the oscilation ranges from around 500kHz to 2 Mhz, with voltages from 4Vpp to 22Vpp.
What I've tried so far:
- different values gate stopper (R115, 1k and 10k), directly soldered to the gate pin.
- different values base-collector (C3, 50pF to 220pF)
- different values speedup cap (C107, 220nF and 3300nF, no difference at all).
With my still limited knowledge, I don't know how to progress from here and what to do to get rid of the oscilations.
Any help is welcome. Thanks
regulatedpsu.jpg
 
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Update: I upped the gate stopper to 47k and the oscilations stopped when using a small load. A heavy load however would still make it oscilate like crazy. A 100k stopper took care of that. Is that normal?? Is 100k a typical value?? It doesn't really inspire confidence, so any insight is welcome.