Problems with serial port interface

[lionelgreenstreet at gmail.com]

Hi,
i'm programming in python for the first time: i want to create a serial port reader. I'm using python3.3 and pyQT4; i'm using also pyserial.
Below a snippet of the code:

class CReader(QThread):
    def start(self, ser, priority = QThread.InheritPriority):
        self.ser = ser
        QThread.start(self, priority)
        self._isRunning = True
        self.numData=0;
      
    def run(self):
        print("Enter Creader")
        while True:
            if self._isRunning:
                try:
                    data = self.ser.read(self.numData)
                    n = self.ser.inWaiting()
                    if n:
                        data = self.ser.read(n)
                        self.emit(SIGNAL("newData(QString)"), data.decode('cp1252', 'ignore'))
                        self.ser.flushInput()
                except:
                    pass
            else:
                return
            
    def stop(self):
        self._isRunning = False
        self.wait()
 
This code seems work well, but i have problems in this test case:

+baud rate:19200
+8/n/1
+data transmitted: 1 byte every 5ms

After 30seconds (more or less) the program crashes: seems a buffer problem, but i'm not really sure. 
What's wrong?
Thanks