I don't have an other ISRs, so this is not a concern.

I do have other stuff in my main loop, but when I receive a character, I want to "suspend" the main loop and complete dealing with the received message which involves receiving more characters, validating the message and sending back an ACK or NAK message, before I resume the main loop. That's what I was trying to do by staying in the interrupt handler until all of the above is complete.

I still don't see what's wrong with this, and I can't see what "debris" can be left.

I'm not "experimenting". This is supposed to be a real solution...

Thanks - Rowan