why does my suzuki gn250 shut off and why dosen't it start up?

[Con]

I own a 1991 suzuki gn250.Starter motor failed to turn over engine, i was told by repair shop that it needed new brushes. I fitted a new starter motor on it but sometime it whizzes and does not turn the engine over. I found that if I shift it into gear it sometime starts up. I also found that when i ride it, the engines sometimes cuts out completely, and so does the bike panel. It stays like that for about 7-10 seconds and then resets itself, I'm then able to start it up again. Ive checked battery leads and most cables....Please help