> But BTFSS/BTFSC never *store* anything back, do they ?

Umm, yes, that is why a bit operation on a Port register can have weird side 
effects on other bits. The whole byte is read, the bit checked, and then the 
whole byte is written back. It does the same operation as a bit set or 
reset, but without doing the set or reset. There was quite a discussion 
about this a good few years ago, and someone came up with some code that 
proved the case.

For the OP, the only way to do what you want is to have a temporary register 
that you read the Rx register into, and THEN do the bit check on the 
temporary register, and if needed transfer the contents of that register 
where you want. 

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