I think my recommendation is still valid unless you have already checked all instances of TMOD, TCON, SCON, and IE byte manipulation, as well as all instances of manipulating the individual bits of TCON, SCON, and IE. Which timer are you using for the baud rate generator? Check for instances where you might be using it for purposes other than baud rate generation. Could it be that other routines (e.g., A/D, display, push-button debounce, etc.) are changing the UART configuration or disabling its interrupts? Check to make sure that anytime your program tests RI or TI for a UART event, that it clears the bit. Keep RI/TI bit tests and clears very "close" to your SBUF reads and writes.