I have not seen a problem with mine, and seeing as the system is not a Nissan system, and is used in other vehicles I doubt the system is "flaky".

It's been getting cold around here (single digits at night) and the only time I had a problem was the first night it reached the teens.

When you say the tire pressure is within specs;
Do you have a decent tire gauge?
what is the tire pressure?
Are you checking the pressure cold?
Are they all above 28psi (cold)?
Are all tires within a couple PSI of each other?
Have you put the spare on or had the spare rotated in during a tire change?