Running the CPU too fast for Vcc (above the curve in Figure 1 of slas504g) will produce "goofy" (pardon the jargon) results -- the debugger will jump around, and/or appear to execute instructions other than the one you're looking at. As OCY suggested, an easy way to get into this state is to erase (0xFF) the DCO calibration constants in InfoA, and then use them. The BSL is particularly good at accomplishing this; it's also possible to do this by incorrect settings in your debugger (mass erase with "erase Info segments", or some such). Looking at 0x10F8-10FF should tell you whether this has happened. There is a program in the sample suite to re-generate the calibration constants, but as I recall it requires an external clock/crystal.
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