Private Delegate Sub DelegateSub()
Private Reci As New DelegateSub(AddressOf Recieve)
Private Send As New DelegateSub(AddressOf Senden)
Dim sendSema As Boolean = False
...
Private Sub SerialPort1_DataReceived(ByVal sender As Object, ByVal e As 
  System.IO.Ports.SerialDataReceivedEventArgs) Handles SerialPort1.DataReceived
 
   portbuf = SerialPort1.ReadExisting()
   Try
        Me.BeginInvoke(Reci)
   Catch ex As Exception
        ...
   End Try
 
End Sub
Private Sub Recieve()
...
   if portbuf = "bla" then
   '''''hier soll die Variable eigentlich zurückgesetzt werden
        sendSema = False
   end if
...
End Sub
......
Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As _
  System.EventArgs) Handles Timer1.Tick
   Me.BeginInvoke(Send)
End Sub
Private Sub Senden()
...
   sendSema = True
   ''''Hier hängt sich mein Programm auf ;(
   While sendSema = True
   End While
...
End Sub