AVR-Chat

i am recieving data serialy from PC to atmega16. i am recieving data 
of four byte frame in avr.then i am setting the avr's ports according 
to data recieved in USART RXC interrupt routine. i is working fine at 
the starting. but after sometime it is mulfunctioning. what should i 
do to resolve the problem.

                    can any body give me its solution.
kindly help me, if you can or if you have any other 
idea................ 
              thanks a lot for it..........