Sounds more like a loose connection.

When you are joining the wires, prior to attaching the wire nut, you need to twist the wires together, however, be sure that all of the conductors are stripped and equal length.

Even though lights have been working fine for weeks and then once cold no longer works

If your connections are as I described, then replacing the switch most used would be the next step.

Heat and one of the switches in a 3 way circuit, left in the midway travel of their throw, will affect a switch circuit far sooner than cold.

Are you throwing the switches the same way all the time? A properly wired 3-way switch configuration will operate regardless of the position of the other switch so that neither has a consistent "on" or "off" position. i.e. if the light is on when you're in the garage, it shouldn't matter if the toggle is up or down, throwing it the other way will switch it off. the other switch should do the same.

An improperly wired 3-way can force a switch to be in one position (say up) in order for the second switch to turn the fixture on and off. If wired improperly and the first switch is down (not up), then the second switch will not do anything and it will seem like the switch is broken.

Is there a chance that you've always followed the same pattern and everything "worked ok" and now the relative switch positions have changed so things "stopped working".

Check the wiring. It may have always been incorrect and you just didn't notice it.

BTW, if you put a modern, quality, common spec ballast in the luminaire with conventional lamps, they should strike down to 0 F. It might save you a lot of aggravation. Either check the specs of the tubes and ballasts or stay away from low output ballasts and low output lamps.

If you need lower temps, you can use a special ballast.