There's nothing to go on in your information to glean what you could do yourself in the circumstances. Frankly it needs to be reported with some before and after evidence because 99.5% sure it's congestion or a circuit fault that hits you during a congested period.
We can try to part-diagnose this for you if you provide us with stats
just before and again when when things are bad. Quite often we can validly suggest a cause for an unstable service based on the data you provide as listed below.
1/
Your modem stats (to include Downstream, Upstream, Operational Config & event log). This is achieved by entering
http://192.168.100.1 in your browser and hitting the Login link, supplying credentials "root" and "root". You then click on each of the menu items I've listed and copy the results into your next post (e.g. by cutting & pasting into Notepad.exe and then copying the lot into the post).
IMPORTANT: REMOVE YOUR IP ADDRESS FROM ANY INFORMATION (You can leave 192.168.xx.xx in).
2/
A speed test preferably using
http://www.virginmedia.com/testmyspeed/manual.php.
3/
Just before modem stats, please do PATHPING
www.bbc.co.uk. Pathping takes a few minutes to run and tells us where congestion might be occurring. If you type
pathping www.bbc.co.uk > c:\fred.txt at the windows command prompt, then you can use
notepad to extract and paste all the information in c:\fred.txt or another location of your choice.
The result of the above would be advice on what to tell the support people when it's called in.
Cheers