Internet randomly dropping off

[#1]

Our connection to the Internet has been dropping off with increasing frequency.
It's now been off most of last night, and was still off this morning when I left, and it's really left me high and dry when I had meeting that I needed to download a file for!
The connection from the laptops or phones to the router is fine, but the light for the connection to the internet goes red and the connection to the internet is dead. happens for a hour here or there, but is getting worse.
I tried the fault checker online, but after confirming our contact details, it came up with:
Broadband Troubleshooter - Error
There's been a problem
We're sorry, but you can't use the Broadband Troubleshooter to report a fault on your service. Please contact our Support Team for help.
I'm at work on my tea break at the moment, but when I called it's a 30minute wait to speak to someone, and I've got to get back to work!
What do I do next?
The random nature of this fault is so frustrating.

[#2]

There were a few details missing from your account that were preventing you from running the troubleshooter. I've added them now so you should be able to run it.
Your connection looks a bit problematic.

[#3]

First thing I would check is when the internet is off does the landline phone still work. Ideally you should use a
corded phone plugged into the test socket with an inline filter, but failing that just try the phone on a quiet line test 17070 option2.
Which exchange are you on and have you noticed any work going on near your cabinet ( green box in the street)

[#4]

Thanks folks, I will try the landline connection this evening when I am back, and when the internet connection drops off again.
Have not noticed any work on green cabinet.
Exchange is:
Exchange name: Inverness Culloden
Exchange code: NSICL

[#5]

OK, so it's been several weeks since we flagged this up, and I'm afraid nothing has changed, if anything it has gotten worse.
Over Christmas it's been much more noticeable, as we've been in the house more rather than being at work, or out.
It's meant that trying to watch stuff online etc has been really frustrating!
I've tried using the landline when the ASDL connection has dropped off, and it is still working fine.
Can't be anyone working on the green cabinets, as it's lasted for so long.
What do I do now?

[#6]

Have spoken to plusnet on phone, fault has been logged and router is now plugged into the hidden port in master socket for 72 hours of monitoring.