8051 Serial Rx Seems Stuck?

Hello, 

I've been tasked with trying to debug someone's 8051 mcu program. The 8051 interfaces with a max232 serial chip to output and receive values from a keyboard and display messages to a computer monitor. The main problem arises when the program will stop responding to keyboard inputs and the same prompt for user input repeats indefinitely after entering any keyboard value. When I modified the program to output what character it thought it was receiving, I noticed strange results. When the program bugs out, it seems to output the same character even though other keys are being pressed. I'm new to the forums and I would like to ask what more information would help paint a clearer picture of my issue? I'm not so sure where the problem may lie. I've thought that it might be a max232 chip issue, or a buffer that gets "stuck". Could it be a stack overflow issue? I added a counter that increments at the beginning of of each function and decrements at the end of a function to track exactly how deep I am calling functions. It never goes above 3, yet the problem persists. Thanks in advance for any replies.