When using the PIC16F818 or PIC16F819 with internal rc clock.
You need to add a line extra for propper working of the device.

Here is an example:

@ DEVICE INTRC_OSC_NOCLKOUT,MCRR_OFF
DEFINE OSC 4
' and now the extra for propper working

OSCCON=$60
'70=8MHz, 60=4MHz, 50=2MHz, 40=1MHz, 30=500kHz, 20=250kHz, 10=125 kHz, 0=31,25 kHz.

When you forget this the divice is not working in PBP