I have a Arduino Duemilanove that uses the HardwareSerial library to talk to external hardware and a lot of times the Arduino just auto reset when I send something on the serial interface. But not always, sometimes it works as it should. I have read a little about the auto reset feature and I have disabled it be removing the RESET-EN wire on the board. I verified that it really was disabled by trying to upload and now I must press the reset button manually for it to start to upload. But my auto reset problem is still there 
. Must I do anything else to disable the auto reset? Or could something else cause the auto reset?
I found the error myself. I was so sure it was the auto reset because when I removed the RX wire everything was okay. But after double checking everything I found that my receiving buffer was too small and I probably ended up over wring stuff on the stack and it was the watchdog that reseted the CPU.
So if someone else have strange reset problem on communication programs, check your buffers! 