... snip snip ...

> The serial input routine is interrupt driven from the RTCC and works fine
> without the sleep instruction.

The sleep instruction effectively stops the oscillator. You can
wake up the PIC by means of external interrupt, but your timing will
obviously be incorrect.

> Can this be done ?

I don't see any reasonable way to implement RS232 using the sleep
instruction. Please, correct me someone if I'm wrong.

Cheers,

RI