Thanks for the suggestions. The serial ISR is only filling a circular buffer by the way.

What I can't understand is that once the uC gets into this state, I never get another receive interrupt on this port. It's as if the RI flag can never be set. Surely even if I miss a character, another character arriving would set the RI bit?? I'm pretty sure the RI interrupt is not edge-triggered (please correct me if I'm wrong).

A further puzzle is that it only takes two characters to lock up the receiver.