What did 'starter serviced' consist of?
Were the bushes replaced? The bushes are the bearings that the stater shaft runs in, if they are worn then the starter will run very slowly under load, especially when the bike is cold.
The easy solution is a new Valeo starter from euromotoelectrics, and if you are really cheap then the generic Valeo copy from the same source.
I don't think that your stater problems have anything to do with your diode board
Also make sure that you choke cables are properly adjusted so the choke works properly.