How bizarre - I was loading the same code into a 18f2550 only yesterday !
Did you change the CONFIG registers in your code as these control the actual OSC speed.

You need to have OSC48 in the code as this is the internal rate the PIC runs at - you set the PLL frequency based on your crystal speed.

Let me fire up the laptop and get the CONFIGs I used for 4MHz and 20MHz which I know work with my breadboard circuit.

bill.