That crystal is correct. It has the proper capacitance and low ESR.
Since these crystals are susceptible to noise I always use a ground
ring. Check out the layout in the datasheet at http://wiblocks.luciani.org/PICO/PICO1TRC-index.html
It sounds like your error is reasonable predictable and occurs after
a long period of time so it is hard to explain this away as noise.
It is good design practice to guard high impedance circuits.
Have you tried running your circuit under more controlled conditions?
Indoors (25C) and off of a bench power supply?
(* jcl *)