Every winter I have the same problem on my 2.0T (2005) it just won't hold 90 degrees solidly.
Ive had the stat changed last year and it was better but still dropped below 90 when it was cold. It was then fine through the summer. As the temps have dropped this year it wasn't holding 90 degrees again so I changed the sensor for a new beru one hoping it would fix it bit it didn't.
It warms upto 90 by about 15 minutes but then drops to 80 even cruising on the motorway at 70.
I could try replacing the stat again, but that's pretty expensive, or get an oem sensor but I doubt thats the issue now.
Any other ideas? Thank you!