Padu,

Your approach of minimizing the time spent in the interrupt routine using 
circular buffers for the UART interface is a good design principle. If 
your Main Loop is unable to keep up, then there are two (or perhaps more) 
fundamental causes. First is what Wouter suggested: the data rate is just 
too high. The second, is that the time spent operating on the data is too 
long. I suspect it's the latter since your serial data rate is only 
19.2kBaud. If so, the design principle I adhere to is one of splitting 
large tasks up into smaller tasks. I usually do this with state machines 
and dispatch tables. For example:

 	MOVLW	HIGH(DispatchTable)
 	MOVWF	PCLATH

 	RLNCF	State,W                 ;Use the State as an index
 	ANDLW	0xF*2                   ;table holds 16 entries
 	ADDLW	LOW(DispatchTable)
 	SKPNC				;
 	 INCF	PCLATH,F		;
 	MOVWF	PCL			;branch to the next state.
DispatchTable:
 	bra	task0
 	bra	task1
 	bra	task2
...
 	bra	task15

Without knowing more about your application, I really can't suggest 
anything more specific. However, I suspect that it's possible to split 
your main loop tasks into state machines and then let the main loop call 
each state machine.

Scott
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