Holy cow... 126°C??? Are you sure your IR meter is measuring fine?
Anything above 100°C is unhealthy and normally an indication that something is not running or set well.
Can you please give me your full settings:
1. gearing
2. motor timing setting
3. rotor you used
4. track size (a link or picture of the track would help)
I can't believe an X12 4.0t is not fast enough with LiPo, that should be insane power on any track...