Electronic – Maximum CPU Frequency of PIC24FJ256GA705 That I Can Get with 8MHz Crystal

I've run into similar issues with other Microchip parts like their real-time clock. In our case the crystal we selected did not meet the Microchip requirements. We saw very similar results in that sometimes it would run fine for a while then stop or run slow all the time.

Many of Microchips new low power designs use very low power oscillators that require very low capacitance crystals. Check the datasheet for your part and crystal to make sure they are compatible. If you still have any doubts check with an applications engineer at Microchip.

The datasheet is correct, as they usually are. The PIC 24F family can run at a maximum instruction cycle rate of 16 MHz, which can be expressed as 16 MIPS (Million Instructions per Second). This processor requires 2x clock over the instruction rate, so the maximum clock is 32 MHz. You can achieve 32 MHz clock with a 8 MHz crystal and the 4x PLL. That will result in 16 MHz instruction clock.