Hi C,
As you know the 18F2431 has these two extra AVdd and AVss [ +V and 0V respectively] pins, many Prommers do not have these two pins internally connected, also pin #26 PGM has to be at 0V.
The option we discussed some time ago was to modify our Prommer's, mine is a PICStart+.
To avoid modifying the Prommer I added the extra links to a 28 turned pin IC socket, I insert the PIC into the socket and then the socket into the prommer, it works every time OK.
I suspect your problems with the 18F2431 are due to your Prommer.
Eric