Seems like the mscomm.rthreshold don't works very fine for high baudrates, then some times when the event is done and the pc goes to read the buffer there are some more received chars in the buffer.

I solved it sending an aditional control char i.e. an "*" or other, seting rthreshold=1, storing the received chars in a string and then search the control char (the * in my case) and then selecting the 4 chars on the left of the control char.

I done it because in my case the strings aren't all of the same length then i can't use a pre defined lenght like yours.

I hope this helps.