How to find the problem on the circuit?

Dear Experts 

 about the fan problem on the motor module , we had the same problem on the 3 motor modules during last 6 years , we checked and we find that all of them worked with firm ware version 2.5   ,it means (I can say from temperature trends )motor module work till about 70 c and after that fans start and bring the temperature to 40 c , we upgrade to 2.6 now(fans work with module enable) , but about damaged ones is there any help for how to find the problem on the circuit , because it's not a permanent problem ,I checked the circuit and it's now OK ,it means we should have a semi defected part on the circuit , please help if any expireance

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Split from Double Motor Module Cooling Fan Problem.