So what is missing when using another PSU ?
Simple : REAL right voltage output...
Todays PSU and PC mobo are too tolerant.
You can have per example a brand new hi-class 420W PSU that deliver +4.68v for +5v... if you got under 4.86v on your Amiga you'll get crash... I tried 4 to one with a correct output...
Amiga machine are not so tolerant, and much less with CSPPC and GREX.
Old PSU got a better output. You can pick up a 5 years old 300W PSU and compare it to the one now, you'll be surprised...
@Keropi : have you tried to power your GREX with both AT and ATX plug ? Sometimes it really help, sometimes it's dangerous...