So it seems sometimes i have to reset to make it work and I wonder why
Yes it sounds like that reset problem especially as you said:-
The power led is on when it doesn't work.
So it is getting power but not running.
sending MIDI data to my synths, if I open the IDE they go crazy,
That is because you are using the same pins for the USB serial connections as you are for the MIDI I/O so your MIDI equipment is getting all sorts of rubbish.
My model is one of the first UNOs i think, it's from 2010
I think that is before they started fitting the diode.