My 98 Acura just started revving really abnormally in cold weather (below 32 deg F). It fluctuates rapidly from 1200 RPM to 1800 RPM for about ten minutes. Is this normal, or is this a sign that something is starting to fail? No check engine codes are found either.

Here's the problem in action:
http://sdrv.ms/LZEQiZ

 

Sounds like its the Idle Air Control Valve.
Go here and download the service manual. http://www.acuralegendwiki.org/index...y_Helm_Manuals
After that go to Fuel and Emissions>Troubleshooting>Symptom Chart
then go to page 11-123 it has the procedure for checking the IAC Thermal valve.

 

My car did that very lightly when cool and we replaced the IAC and fast idle mechanism and it appeared to have stopped but in reality the weather had gotten warm. All summer it idled fine until one day it hit 50F and it came back. When it was under 32F it did it more pronounced. They flushed and bled the radiator a few times then it stopped. Air bubbles in coolant can also cause the idle surging. I did a search and it seems this is not a rare thing for Acuras but no solutions were posted.