So, I've been playing around a little longer as something started acting up again. I may have been too hasty posting my work around (apologies for that)
I have a PAL model 1 with bios v3.4 (only hang-on built in), for the record I did the switched version of the 50/60hz mod.
So far I've discovered if I try booting the cart in 60hz mode I get a 100% failure rate "Software Error", however, booting 60hz to hang-on works fine.
Then I "thought", when I boot the cart in 50hz, the first time it fails, if I turn off then on again with-in 5 seconds it boots fine. If I then turn off the system for more than 5 seconds and then on it fails again. This turns out to be inconsistent.
Just a thought, fille1976, what is the power rating of your power supply? 10v, 9v, 850ma, 1.2a? Mines the standard 9v 850ma, I see the U.S adapter is 9v 1a. Could that be our issue???