Sounds like you may be too lean, have you checked for air leaks?

When running in cold weather you'll need to wrap some alum foil around the head to keep temps up....could be that you're too lean because you're tuning by temp...and the head is not holding enough heat, giving you erratic condition

Cold air is much more dense then warm, leading to a lean condition.

Detonation may be causing the plug failures.

Try the OS LC3, they are hot, and longer than the #8,... oh yeah and richen it up!