Given the way you have described the condition, I come to the following conclusion. In warmer weather you do not warm up your engine on the stand, therefore; the packing last longer. Yes? In colder weather, you warm it up on the stand and in so doing unburned fuel is dumped into the exhaust pipe soaking the packing. When you finally go full tilt, it burns it.
Solution: Try using/adding the Steel Exhaust Isolating Mat. Also, if you start your engine about 2 minutes before you roll out, you should be up to temp within a lap especially if you put your hand over the radiator. If too cold, add tape. Starting it on the stand should then be for making sure all system are a go.