1) JNB TI, $ ; Wait till done transmitting 

2)CJNE R1,#BUFFER+BUFFER_SIZE,MAIN_LOOP ; if(R1 == BUFFER+BUFFER_SIZE) 
MOV R1,#BUFFER ; R1 == BUFFER

1) should be dropped it will hang there
2) should be in both ISRs

another comment:
make the buffers a power of 2 e.g. 16 long and start them on a power of 2 boundary.

Assuming buffer start at 080h and length = 16

in the isr

inc Ri
anl Ri,#0f0h


takes care of it.

Erik