Test 1, slow 16F877 to 4 MHz:

Code:
#CONFIG
    __config _XT_OSC & _WDT_OFF & _WRT_OFF & _BOREN_ON & _LVP_OFF  & _CPD_OFF & _PWRTE_ON & _DEBUG_OFF
#ENDCONFIG

DEFINE  OSC 4
Lab-X1 jumper A=23

Nothing...


Go back to HS @ 20MHz with jumper A=12:

Back to twice as fast as it should...

Really puzzled, I expected it to run perfectly at 4 MHz.