Microchip seems to agree. From their support:

-------------
Read-modify-write can cause that.  I strongly recommend never bit setting or 
clearing a port.  You should always read the port, copy it to ram, modify the 
bit, then write that back out to the whole port.  Since the PIC is trying to 
change all 8 bits (even if some are the same), this prevents it from reading 
the other bits and changing them.  You can learn more about read-modify-write 
by searching our knowledge base.

The PIC18 gets around this by having an output "LAT"ch register.  If you bit 
set LATB,3 for example, then only that pins output register is set, the others 
are not read from the outside world or modified.

Finally, watch out for capacitive loading on pins.  If a pin is strongly 
capacitive, it could be read incorrectly due to the capacitance holding the pin 
state.  When read incorrectly, it is also written incorrectly.  Again, using 
the method mentioned above should assist with this issue as well.
--------------


Wouter van Ooijen wrote:
> 
> I definitely do advocate using shadow registers. 
-- 
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