Motor heat issue

I have a Novak HV 6.5 with 12 cells in my revo truck and the temperature of the motor is rarely above 122F.
However, when I change the HV 6.5 to a Quark 125 esc and a C50 maxx the temperature of the motor is always an issue.
With the same voltage and gearing of HV 6.5(72/15 of E maxx transmission), the motor easily reaches 140F for about 2 min speedy run .
Even when I change the motor to a 10L kv:2450 motor, the motor is still running hot.
No matter the pinion comes from 14 to 17 tooth, the temperature is always above 140F for about 2 min and even 158F after 2 min.
I couldn't imagine if I exhaust a set of battery, how hot the motor would be.
Are you guys running your motor so hot?
Any suggestion? Thank you.

ps. Ambient temperature is about 90F. The esc is rarely above 122F.