First replace the power supply with a new one. It is a cheap experiment. If with a new power supply continues to reboot frequently, then you may have either a hardware problem in your 901N or you may have firmware problems. In this case reflash the latest firmware and experiment. If you still have frequent reboots then you are done. Your unit has hardware fault. Replace it completely.

Thanks for the reply but as I have mentioned Firmware is updated Bita version. Is there any need to replace firmware with stable TL-WA901ND_V2_120224 or stick with TL-WA901ND_V2_130131_beta?

Unless you receive a very well technically documented answer for not trying other or older firmware versions, you can try other versions. Sometimes reflashing the existing firmware with the same version firmware fixes odd problems like yours. You must balance the risks of bricking the device with the benefits you will get by possibly fixing the problem.

So, as of now what should I do for trouble shooting for the core reason of restarting of anything happening with restarting?