Agree with everything suggested above...
If it's doing it the moment you’re dropping it into gear then its electrical problems with the safety kill circuit that bikes including yours has.
If the problem occurs with just the bike sat there in neutral then you gotta start looking at problems with either the air/fuel or ignition...including faulty rec/reg leaking AC into the DC circuit of the bike.
Good way to check for intake seal leaks is spray WD40 round them, if it gets sucked in when the engines running you got a leak
When I had a 400 it was one of the most reliable bikes I ever owned (and used daily everywhere), so don't give in once you got it sorted it will be solid for years.
