I had a similar experience after using Risolone coolant flush. After bringing temp up to about 160 degrees,idle went and stayed around 2500 to 3000 RPM. Repeating shutdown and restart again and again,I immediately let cool a bit and drained/flushed entire system and restarted with plain water. I did this 3 times before normal RPM operation returned with clean anti freeze. The wax unit must be very sensitive I concluded especially after reading this post. Scared the living crap out of me. This may not pertain exactly to this post,but has some similarities.