Just last week when it hit minus -1C I noticed it took forever to reach operating temp (middle of gauge). When it finally did, it seemed to hover between dead center and a little to the left. I removed the fan, and it didn't help. Then I replaced the t-stat with an OEM and it heats up very quickly now, compared to before.
The old t-stat was an unoriginal part I installed last April, when I swapped engine. The buildup of yellow residue is also gone now
Funny thing is that I didn't notice any of this until it got pretty cold.
"If you find yourself in a hole, the first thing to do is stop digging." - Will Rogers