GPS-Geraet streikt

Hi,

ich habe mir einen kleinen GPS-Empfaenger gebastelt. Komischerweise 
resetet sich das ding in etwa jede Sekunde und ab und zu funktioniert es 
per Zufall, kann mir jemand weiterhelfen? Hier der Sourcecode:

$regfile = "m8def.dat"
$crystal = 16000000
$baud = 4800

Config Portb = Output
Config Portd = Input

Config Lcdpin = Pin , Db4 = Portc.2 , Db5 = Portc.3 , Db6 = Portc.4 , 
Db7 = Portc.5 , E = Portc.1 , Rs = Portc.0
Config Lcd = 20 * 4

Cls
Cursor Off

Dim C As Byte
Dim I As Byte
Dim Time As String * 6
Dim Av As String * 2
Dim Lat As String * 8
Dim Ns As String * 2
Dim Lon As String * 9
Dim Ew As String * 2
Dim Spd As String * 6
Dim Crs As String * 6
Dim Leng As Byte
Dim Xx As Bit

Dim A As Byte

Declare Sub Wait_for_string(byval S As String)
Declare Function Read_string() As String

Cls

Wait 1
   Upperline
   Lcd ""
   Lowerline
   Lcd ""
   Thirdline
   Lcd ""
   Fourthline
   Lcd ""
Wait 3

Cls

Config Serialin = Buffered , Size = 100                     ' Konfig. 
der ser. Eingabe
Enable Interrupts
' $GPRMC,hhmmss.ss,A,llll.ll,a,yyyyy.yy,a,x.x,x.x,ddmmyy,x.x,a*hh
Do
   Call Wait_for_string( "$GPRMC,")                         ' Warte auf 
$GPRMC

   Time = Read_string()
   Av = Read_string()

   Lat = Read_string()
   Leng = Len(lat) - 1
   Lat = Left(lat , Leng)

   Ns = Read_string()
   Leng = Len(ns) - 1
   Ns = Left(ns , Leng)

   Lon = Read_string()
   Leng = Len(lon) - 1
   Lon = Left(lon , Leng)

   Ew = Read_string()
   Leng = Len(ew) - 1
   Ew = Left(ew , Leng)

   Spd = Read_string()
   Leng = Len(spd) - 1
   Spd = Left(spd , Leng)

   Crs = Read_string()
   Leng = Len(crs) - 1
   Crs = Left(crs , Leng)

   If Av = "A," Then
   Locate 1 , 1
   Lcd "LAT: "
   Locate 2 , 1
   Lcd "LON: "
   Locate 3 , 1
   Lcd "HDG: "
   Locate 4 , 1
   Lcd "SPD:"



   Locate 1 , 6
   Lcd Lat
   Lcd Ns

   Locate 2 , 6
   Lcd Lon
   Lcd Ew


   Locate 3 , 6
   Lcd Crs

   Locate 4 , 6
   Lcd Spd

   Locate 1 , 16
   Lcd "    "
   Locate 2 , 16
   Lcd "    "
   Locate 3 , 11
   Lcd "          "
   Locate 4 , 11
   Lcd "          "
   Locate 4 , 5
   Lcd " "






   'Spdkm = Val(spd)
   'Spdkm = Spdkm * 1852
   'Lcd Spdkm


   'If Av = "V," Then Av = "*"
   'If Av = "A," Then Av = " "
   'Locate 4 , 11
   'Lcd Av
   End If

   Toggle Xx

Loop

End

'------------------------------------------------------
Sub Wait_for_string(byval S As String) As String
Local Ii As Byte
Local Cc As Byte
Ii = 1
M1:
   Cc = Waitkey()
   If Cc <> Mid(s , Ii , 1) Then
      Goto M1
   Else
      Incr Ii
      If Ii > Len(s) Then Goto M2
      Goto M1
   End If
M2:
End Sub

'------------------------------------------------------
Function Read_string() As String
Local Cc As Byte
Local Ss As String * 10
Ss = ""
Do
   Cc = Waitkey()
   Ss = Ss + Chr(cc)
Loop Until Cc = ","
Read_string = Left(ss , 10)
End Function

'_____________________________________________________