vico,

Just a thought, sometimes a #pragma NOAREGS in the interrupt section might help. Can you try that? Due to the switching of the reg bank, the compiler using Absolute register addressing can muck up the code.

Another aside, from your statement that keeping the priority same for the Timer0 and Serial ints solves your problem, I suppose, the serial int is repeating faster than the timer0 int and causing a stack pileup resulting in a freeze. You may like to check the t_disabled bit as it seems to be doing nothing in the transmit section of your serial interrupt code.

Third possibility (not directly related to your code) is that an interrupt routine calls a function that is also called in the mainline program. Perhaps this function is not marked as re-entrant?

Hope this helps a bit

Jerson