A smidge above half is over 200 degrees F, I'd suggest a complete flush and a new 185 degree thermostat.
Make sure you use distilled water and the proper antifreeze.
I used Zerez G05, 1.5 US gal and the remainder distilled water.
The thermostat was around $45, easy to change.

http://www.cincinnaticomm.com/james1...lantflush.html