1. Try adding 1.0uF caps to the input and output of the 5V regulator as well.
  2. I would make sure the 4.7K pullup on the MCLR line has a solid connection. A MCLR left floating would also cause intermittent resets.
  3. Or just disable the MCLR function (MCLR_OFF, if I remember correctly) just to be sure some unseen noise spikes are not triggering the uC reset.