Hope someone can help me!

So I have been having an issue with my broadband connection (Xtra) whereby over the last few weeks, on about half the days my brother or I have tried to play an online game (Team Fortress 2, Heroes of Newerth, Starcraft 2) the internet connection will freeze up for around 3-5 seconds at a time every few minutes, for hours (normally we just give up as soon as it starts, but yesterday I persevered for 2 hours before I gave up, just to see how long it would go for).  We aren't dropped from the game, but we lose control for those seconds while the game goes on.  Also having some issues on Modern Warfare 3 on PS3. Needless to say that can be the difference between life and death.

We use 2 different PC's, one laptop over WiFi, the other a desktop over Ethernet, and both have the same issues.  I have tried different jackpoints, filters etc but no use.  Haven't tried another modem, as I don't have a spare.  But given it is terrible some days, and fine others, I am thinking it is more likely an issue with my connection than the modem itself?

I called the helpdesk, and they said everything looks fine and hopefully it goes away. So no joy there.

What do you guys think may be the issue, and what can I do to fix it?