remote con. lost, winCE Visual studio

hi all,
i have a real big problem, i dont understand!

programm overviw:
all threads with realtime priority! (thread 1+2 priority 15, thread 3 priority 16)
2 threads receiving from a CAN-Bus
a 3rd thread starts periodically and wants to write a textfile.

the 3rd thread is something like this:

while (x==0)
{
//fill a char buffer
writeFile(charbuffer)
if(whatever)
{x=1;}
}

Problem: While the 3rd Thread is running i get a error from Visual studio: "The remote connection to the device has been lost"
This happens sometimes on the 1st time it run, sometimes on the 2nd time...

one think solves the problem nearly, if i call the OutputDebugString() function alter WriteFile() , to give me debug msg, the error happens on the 10-100 time the thread starts.
I'am really desperate atm. sitting here for about 20h and trying to solve this :(
does someone know something?

how is it possible that a simple debug msg. can reduce this error that hard?