Jon,

This just shows how stupid showoff I am.
It can't work properly EXCEPT the value of 4...
(This happened the same way in the "code competition" - Maarten Brock noticed the error in what I thought is perfect solution and for testing I chose one of the rare cases where it really worked...).
The trouble is in the calculation of how many characters are in the receive buffer - GET_RX_NR - in case the "head" is "below" the "tail" (head wrapped around but tail not) it shall not get the complement but shall add the size of buffer instead.
The corrected GET_RX_NR:

GET_RX_NR:
   CLR   C
   MOV   A,RX_HEAD
   SUBB  A,RX_TAIL
   JNC   GET_RX_NR_X1
   ADD   A,#RX_BUFF_END-RX_BUFF+1
GET_RX_NR_X1:
   RET

I will try to do the comments asap.

Jan Waclawek