Hi owali,

I could not find any reference to the PIC16F83A.

If you PIC can allows a 20Mhz resonator try:
DEFINE OSC 20

Also, the PBP manual says;
The oscillator defaults to XT on most devices.
This is the setting for the default 4MHz oscillator.
If a faster oscillator is used, this setting must be changed to HS.

-Adam-