Do you have MCLR tied to +5V directly or have you used a resistor ?

Or have you done what I did on a protoype board and forgotten it. My board had been in used for months as I was developing code and was working fine. I kept adding bits of code and other circuitry and then one day started getting spurious random resets.

I had forgotten a pull up on the MCLR pin and it was now triggering on noise picked up from the ratsnest of prototype wiring. fitted a 1K resistor and it has been fine since