Rupert,

might be a long shot, but try the initialisation in a different order. I
had the experience once of odd I/O behaviour with an F88. The
correct commands were all there, according to the manual, but in
the order I'd written them the chip didn't initialise properly. Once
that order was changed the PIC worked perfectly

I'd give you another example. The controller for the DOGM LCD
doesn't work unless a specific sequence and timing (and not the
same as is in the datasheet) is followed

http://www.piclist.com/techref/microchip/io/dev/lcd/DOGM.htm

Be wary of code examples in PIC datasheets. They can lead you
seriously astray as they are often pasted from other datasheets

I might suggest changing the clock source too, or at least try another
one temporarily. I've had issues with IntOsc, particularly 8Mhz on
some PICs. For example, bits in module registers don't work until
the speed is dropped to at most 4MHz

Never had issues with crystal clocks though. Perhaps try that as the
'standard' and see if you still have a problem

Joe

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