You always want to clear interrupt flag bits before re-enabling interrupts.
RCIF is the serial receive interrupt flag.

Since RCIF is read-only, you need to read RCREG until it's empty to clear
RCIF. You'll also want to clear any over-run conditions.

Code:
OERR VAR RCSTA.1    ' Alias USART over-run bit
CREN VAR RCSTA.4    ' Alias USART continuous receive enable bit
RCIF VAR PIR1.5	    ' Alias USART received character interrupt flag bit
Try adding this to your BASIC interrupt routine.
Code:
     DISABLE
indirizzo:
     PORTB.7 = PORTB.7 ^ 1   ' Toggle LED on entry to int handler
     IF OERR = 1 THEN   ' If over-run, then clear it
        CREN = 0   ' Disable receive
        CREN = 1   ' Re-enable & clear over-run condition
     ENDIF
     WHILE RCIF   ' If RCIF is set, then read RCREG until it's clear
        X = RCREG
     WEND
     RESUME
     ENABLE