hi mate, your problem sounds like a bad earth in my opinion, your loom will have lots of looped feeds and earths on it and if one of these isnt doing its job this could be causing your problem. ive had this problem on a few of my bikes in the past but only realising after ive spent a fortune on cdi units and rectifiers. im not saying this is guaranteed to be your problem but it would be worth a look. hope it goes well for you