Microcontroller (AT91SAM7A3) only crashes when not running with gdb

Hey guys,

I am debugging some code on a AT91SAM7A3 that shows an odd behaviour: as 
long as it is connected by JTAG and run via gdb, everything works fine. 
If I do a power-on reset without running gdb, it boots up fine, but I 
can make it crash by sending some characters on a USART port (what works 
fine while debugging). The crash is 100% repeatable.

That puts me in a bad position because it's kind of hard to hunt down a 
bug that only shows up when the debugger is not connected.

It's a bare-metal system, the code is C and C++, the compiler is 
gcc/g++.

Has anyone experienced anything like this before?

best regards,
Jan