Ok i am stupid as i cannot track what is wrong.
I finished my mb-sid llong time ago. However this is the first day i plugged it on power ;D
So when i leave it on press nothing it reboots out of nothing. And the time it does it is not consistent?
Is it my c64 power?
Still i have a feeling it’s a shortage but where?
Point me in the right direction 
thx in advance
I’d start by taking all the sid chips out to be safe…
Does the LCD light up and/or show characters? how about the LEDs?
Does the regulator get hot? can you measure the voltage at the regulator and see if it drops out when the core reboots? Are the elko caps in the right way?
Do you get any midi messages or sound?
It sounds like a brownout reset, or maybe overheating (which could be a short).
I would suspect the power supply. I had issues with a box I built a while ago, It would work fine, and when I lit a certain amount of leds, it would reboot. Interesting thing was, some outlets would work fine and others it would reboot. So I know this indicates weird power in the house, but I think it was more that the PS was very crappy and sensitive, I got a higher quality one and the problem totally went away.
Luke
Nope regulator does not get hot. Caps are all good posistioned. Led looks normal. Booting normal can even make a patch etc.
I will see if i have another psu laying around
Yep that sounds very much like a brownout (both of your machines)
Good luck twinny!
Sounds like 5V supply is having trouble.
Try running it with only two SIDs, which will be far less load and in theory the C64 supply can handle this and not “brownout” (drop below 5V).
Brown Out Reset at 4.5V
But yeh… <5V is not OK.
I have not mesured anything yet. But i might have found the issue. I think a crystal is shorting out. (time to insert the protectors and then look further.)
Thanks so far everyone i will update when i have adressed this issue.