Question

P0870 Chrysler PT Cruiser 2005:

Vehicle has no other apparent problems. Gear shift is still smooth. This fault code appears at random. 3-times in 3-months. Fluid-levels checked and 4000 mile journey completed with no fault; but then it happened as I pulled away. Cleared Code (after repeated check by mechanics) fault re-occurred after 2-weeks (1600 miles).

What could cause something so random?