>After a number of frustrating hours and much fiddling, I discovered that the
> problem occured only when the
>program reached 256 bytes.

If you use a table, or anything that does a "addwf PCL", you must make sure
that PCLATH is set correctly. For most programs without a lot of tables,
just making sure your tables are within the first 256 bytes is enough. You
will never need to worry about PCLATH.

If you have a large table, that ends up being a problem, because a table
can't cross a page boundary, meaning tables must be less than 256 bytes in size.

If you have a table that is in a page other than the first 256 byte page,
you must set PCLATH before calling the table, as in:

        movlw   1       ;this is the upper byte of address 100h
        movwf   PCLATH
        call    MyTable


        org     0x0100  ;this places the table at exactly address 100h
MyTable
        addwf   PCL
        retlw   number
        retlw   anothernumber
        retlw   etc....

This isn't complete, but you'll probably get a few more replies to your
question, and between this, that, and the 16c84 datasheets, you'll see what
is happening.

Cheers, Bob