@Sunshine 

Thank you for the recommendations. Yesterday the router worked fine without any hiccups.

I have gone through the article and will try the suggestion to change the DNS settings if the problem re-surfaces and update the post.

Thank you

  @Rubensds68 

This may sound silly but without knowing the environment the router is in it could just be getting too hot.

If you can try to remember what the room temp is when you have issues vs a clear day, what I'm trying to rule out for you is that on the day it played up assuming you are in a hot climate the next day the temps had dropped allowing the router to not have to work so hard or detect it's about to overheat. Also just incase and you aren't in a hot climate ensure that the router has space to breath, lots of people bury their router in a heap of other things which also makes them warm up too much.

It could also be others factors like is it near a microwave oven or other 2.4 ghz devices giving out signals, this will beat a router wifi into submission so maybe moving it somewhere clear in the property may help, the final one that causes these issues outside of a duff router is the cable from the router breaking down, an outside chance if you live where electricity supply fluctuates a lot this could also be causing issues, does it do it at peak times? i.e when everyone gets home and are plugged in.

I just had some more possibilities, is the 6000 standing alone or is it tagged to the isp unit?

Assuming it's tagged check these things, make sure both units have different 192.168 addresses, turn off the wifi on the isp unit, check the MTU value is correct for your isp (outside chance it's wrong) 

None of this may apply but Sunshine has too many variables to look at to give you a conclusive answer, check things to eliminate potential culprits, if none of that works it could just be faulty.

@Bogle

Hi Bogle, the router is located in a study on top of a book cabinet and the ceiling is fairly high - far away from microwave. I reside in Johannesburg, South Africa, and indeed this time of the year we experience hot temperatures. The router sits on top of mesh wired frame type tray so it has plenty of space to breath and the cable to connect to the ISP FTTH box is in very good condition.

I am not sure what you mean by this: "I just had some more possibilities, is the 6000 standing alone or is it tagged to the isp unit?"?. The router is connected to the ISPs FTTH box using the standard 2.5 Giga WAN Port. Whenever the devices lose connection to the Internet I check the LEDs on the FTTH box to see if it is a problem on the ISPs end. In most cases, the WAN LED on the FTTH is on indicating the line is up. 

Assuming it's tagged check these things, make sure both units have different 192.168 addresses, turn off the wifi on the isp unit, check the MTU value is correct for your isp (outside chance it's wrong)

I do not know what the IP settings are on the ISPs FTTH box, as a matter of fact, I have never tried to access it.

I use the standard MTU values from the ISP.

To date, since my original post, router is behaving really well.

Thank you for the suggestions, will keep monitoring and provide feedback.

  @Rubensds68 

The tagged to bit is if you were slaving the 6000 to an isp router connected to the FTTP ONT, now I can see you aren't none of that or changing the 192.168.xxx.xx settings apply.

I'd still monitor it heat wise as they do get hot and they will trip out if they reach too hot status, easiest way is to simply put your hand on it and if it's really hot that is probably why you're having problems as if the router was faulty it would be constantly dropping you a lot more often.

When I had the VR routers for ADSL I would always stand them up vertically so they had the best natural ventilation route the routers could provide, with the 6000 iirc it's a flat square unit so something you might want to try if it has air intakes in it's base is to simply put a couple of supports under it to get the base an inch or so off of any contact surface below it.