You got me thinking; you are using an old PIC. The PIC itself has a part in driving the oscillator crystal. Would a newer PIC, with "better" technology be more accurate than 125 ppm?? I have a time-critical project I'm using a PIC18F27Q43 with a 16 MHz crystal to generate a 64 MHz Fosc (w/PLL). It seems to be as accurate as my scope can show. Just thinking out loud.