so just to be clear. If you run a leJOS program on the nxt (i.e. you download and run the program), that loops forever updating the screen with say the current time, then it will shutdown after 5 minutes? Have you actually tried doing exactly this?
There is no timeout logic in the firmware so I would be very surprised if the NXT shuts down in the above case. The only timeout logic is in the leJOS menu program (which is why it may shut down if you are running a program on your PC that controls the NXT, remotely)....
Hmm. I think I remember something like this when I was just starting out, I had a program for a car that would drive motors forever (I would just be changing speed to control it.) After about 5 mins, the brick would shut off. Unfortunately I can't find the thread on it to tell you why, but I would suggest running a loop test (something similar to what Andy said,) as the motor operator might be different. Someone else will know more than I
When you say fails... Do you mean that the device is gets powered off. I assume you can simply restart it by pressing the orange key when it is in that state? I just want to check to see if it has powered off or has got locked up in some way...
I've just had three NXTs (2 with Lego rechargeable batteries and one with normal batteries) running for about an hour (at which point I turned them off). So I don't think this is a general problem. What type of batteries are you using? The NXT does have a thermal fuse device to protect it, I wonder if your NXT is faulty and is overheating this?