Okay I know this may sound stupid....(and may earn me the winner of the STFU award), but doesn't rubber shrink a wee bit in cold weather?? It could be the slight shrinkage that's causing the clunking sound. The plastic spacers that you have might not be able to compensate for that shrinkage. You said this only happens in the wintertime & not during the warmer fall, spring & summer months. That might be the reason for it...Try switching over to another type of subframe spacers.
Anyhow..the reason I came up with this stupid theory is because people often say when installing any type of bushings to throw them in the freezer so they shrink a little, making the install a little easier.
Anyhow, feel free to let me know if I'm full of it or not......
ID
..winner of the
award...