These fans are not made to be run all the time, but thanks to our mechanics who make them direct they run all the time. This weakens the motor.
There is a possibility that your motor may be weak but my opinion would be to try all the other possibilities told by the fellow pakwheelers and still if the problem continues then change the motor.
Also another thing, if the fan is not having a relay then install a relay, it will increase the speed of the fan.