Check its power supply. These things have a tendency of caps failing inside them, causing the DC output being less and less perfect DC. At some point, the router itself becomes unreliable because of the unreliable current it gets. 

@trance 

Observe to see if the router gets hot or any heat issue. Next, try reducing the WiFi Transmit Power to low and see if the router is any cooler. Furthermore, turn off the WiFi signal all together and observe if the router is cooler than before.

I have an Archer C1200 V1, and noticed the router was very hot. Yes, the router rebooted every so often. I had to use a USB fan to cool the router. However, I decided to investigate further why this is. First, I reduced the WiFi Transmit Power to low and noticed the router is cooler. Next, I turned off the WiFi completely and the router is much cooler. Finally, I reset the Transmit Power back to high and sure enough, the heat was tremendous and a USB fan is needed to cool the router (using the router USB port).

Additionally, the Archer C1200 V3 has a newer firmware, released on 5/13/2020. Possibly ask tp-link support for firmware release for V1 and V2.

https://www.tp-link.com/us/support/download/archer-c1200/v3/#Firmware

I hope the above helps you troubleshoot the issue.
 

@Tommi 

Observe to see if the router gets hot or any heat issue. Next, try reducing the WiFi Transmit Power to low and see if the router is any cooler. Furthermore, turn off the WiFi signal all together and observe if the router is cooler than before.

I have an Archer C1200 V1, and noticed the router was very hot. Yes, the router rebooted every so often. I had to use a USB fan to cool the router. However, I decided to investigate further why this is. First, I reduced the WiFi Transmit Power to low and noticed the router is cooler. Next, I turned off the WiFi completely and the router is much cooler. Finally, I reset the Transmit Power back to high and sure enough, the heat was tremendous and a USB fan is needed to cool the router (using the router USB port).

Additionally, the Archer C1200 V3 has a newer firmware, released on 5/13/2020. Possibly ask tp-link support for firmware release for V1 and V2.

https://www.tp-link.com/us/support/download/archer-c1200/v3/#Firmware

I hope the above helps you troubleshoot the issue.

@trance How do we contact TP Link support to have latest firmware for V2? By the way a Beta Firmware seems to have worked for some users. You can try it:

https://static.tp-link.com/2020/202001/20200103/Archer%20C1200(EU)_V2_180308(Beta).zip 

I do encounter the rebooting problem myself.

Can't figure out what is going on. I even reset to factory default, but still have the issue.

I tried to figure out what went wrong or what is different then before, but cannot find anything.

About 2 weeks ago I had a solid router that did his job.

Firmware: 1.1.3 Build 20180124 rel.52299 (EU)

Hardware: Archer C1200 v1.0

I hope you can help me, because it is very annoying and very bad since I need a stable connection for work.

@MichaelR try using the beta software here:

https://static.tp-link.com/2020/202001/20200103/Archer%20C1200(EU)_V2_180308(Beta).zip 

ever since I am on this firmware, my router does not reboot. Plz tey and let me know if it helps. 

@SKHERE4U 

Thanks,

When i login on tplinkwifi.com and try to upgrade the firmware I downloaded from the link.. I get an error saying it cannot install.

It says V2 in the URL, think it cannot install since I have an V1 hardware

@MichaelR can you try this:

https://static.tp-link.com/2018/201801/20180129/Archer%20C1200(EU)_V1_180124.zip

You can check V1 versions here:

https://www.tp-link.com/in/support/download/archer-c1200/v1/#Firmware