Concentrator not reachable

[#1]

5 times in total (in one month of service), and twice in the past two days my net has gone down for approximately 15-20minutes. Each time with the error "Connection could not be established: Concentrator not reachable" on the router home page.
I normally restart my router and modern after seeing this but it doesn't do much to help.
Any reason why this happens? Is it a problem with plusnet or BTopenreach? 

[#2]

Hi there,
I've just run a few checks for you but can't really see anything obvious as a cause for this so I'd advise raising a fault: http://faults.plus.net

Here's a log of your connections/disconnections over the past week.
<img src="http://ccgi.psmith12.plus.com/visradius/generated/image13548730145981.png"/><div style="clear:both;"></div>

[#3]

Ok, I will do, thanks.
The weird thing is I still had a default gateway IP during the outages, which might explain why it's not showing up on that log.

[#4]

I had this error on two seperate occasions. Each time it was fixed by resetting the router using the pin hole method. The second time I asked for a new modem and haven't had the problem since.