Maybe it was the climate where you live. Here in Australia, mine would definitely get to operating temp, it would just take a long time. And if you sat still long enough, it would eventually get up to operating temp.

When I have previously had a bubble in the system in other cars, sometimes you can hear bubbling upon acceleration or deceleration. Usually that sound sounds like it's coming from around the heater core somewhere. Also I've only ever had bubbles in the cooling system after a coolant flush and replacement. I'm not sure if it's possible to get a bubble in the system just through normal operation?

Either way my guess is a thermostat esp. if you just bought the car because often the previous owner wouldn't have noticed / cared if the temp wasn't getting above halfway.

Also I had a thermostat issue when I bought my car and I noticed on the previous service paperwork for my car there was a mention of a 'faulty coolant sensor' which never got fixed but I think it was the mechanic misdiagnosing the faulty thermostat.

Fyi - Also here in Australia a genuine Audi thermostat costs $92AUD!

 

Could of been my climate. It was 20 Fahrenheit when I was having the problem, but it heats up quick when it working properly. My drive to work is ~4 miles and it always reaches the middle. Before it wouldn't even move, even after 30 minutes it wouldn't be in the middle.

I paid $11 USD for my thermostat. The coolant is $20 a gal though, thankfully I had some already.

 

Thanks for the help! I changed the thermostat and everything is fine now