@Mr.p I recently started facing this issue, The TP link tech support here in India is absolutely useless and have been of no help. In my quest to solve the issue I tried setting time to manual, i tried disabling LED, I have disconnected devices, reinstalled firmware, factory reset and everything else that i thought might be causing the issue, but to avail. 

Is there a solution yet. I am on Archer C9 v5 with the latest firmware. This is getting really annoying and my router isnt even 2 months old. 

@Bireshwar hello ... what is happening? It keeps on restarting ?? even if only the power cable is plugged in?

It would be great to get some response from TP-LINK on this matter. My router is about 2 months old and ive experienced this exact issue on latest firmware ever since i got it.

@Mr.p I've got this exact problem. It only started occurring this week. A real pain in the arse. Anyone have a solution yet?

@Leesy  - Double and triple check the power cord for the device. In particular, make sure that you didn't inadvertently switch the cable with another device. I found I could reliably reproduce the problem by using the wrong cable. Upon further review, I had switched them when I did some cleaning in my office. The "wring" cable was outputting less amperage. It was terribly annoying since the device ran fine when you didn't try to do anything requiring bandwidth. Once you fired up a conference call, it would restart itself.

@nwnomad Im using the power cable supplied with the device. However, I livechatted with the TPLink techs this morning who advised to switch outlet to a different power socket. I did that - cant see what difference it would make tbh - but no joy. Just had a restart again a few minutes ago....grrr.....

So my power is 12v 2amp....

@Leesy I started having the exact same problem about 2-3 days ago... It's driving me crazy.

@lost_guy yeah me too. Only started 2-3 days ago too. I've tried everything including turning odd the LED lights as all th errors seem to relate to the leds. Bizarre that it should happen around same time to other owners. Maybe they need to write a new firmware release. 

Here's what the tech team advised. I've done this and so far....all good. But I'll reserve judgement until a few days have passed. 
 

Just an update to everyone facing this problem. Apparently this is rampant with no solution from TP LINK themselves which is quite disheartening. Anyway, this kept happening so much that i had to revert to my old router as primary. I connected a separate line to this TP LINK one and let it be.. Sure enough the restarts kept happening every 15 minutes. Then i tried installing the previous firmware on it, and lo and behold the restarts disappeared. To check whether it is working or not.. I kept it on and as backup for 2 weeks and not a single restart. I switched it back to primary and it has been working fine. Sometimes it just starts randomly restarting but reverting or upgrading between the 3 available firmwares seems to work so far and has kept me from chucking this router out the window. 

I am in no way claiming that this fixes it permanently, but it sure does stops the annoying restarts for a varied period of time. So everyone who has been facing this.. Try installing a previous firmware and if it fixes it then great, if it still restarts, just switch to another firmware.

Let me know if it helps. 

Cheers.