Melanie:

I use the internal oscillator in many applications so I could have the CLKIN & CLKOUT available as I/O pins, especially when using the 8pin (12F) and the 6pin (10F) versions! "Accidentally" erasing the chip before programming and using the define OSCCAL_xK 1 statement can send the chip to the la-la land.....

You know a lot about the internal harware config but modifying the oscillator value with 255 will certainly be way out from a value with a number closer to 100.

Reading a new chip and getting all 3FFFh value means the chip has been erased and not "fresh". As I understand it the last position without the 34xxh is the only difference between a new chip and an erased or programmed chip......

-warrier