Severe annoying online issue

first, sorry if this is in the wrong section but I'm not sure if it's related to my networking, which is fairly new, or a SW issue. So please move it if necessary.
But this is a VERY frustrating issue I have had since building my new PC from the LGA775 platform over the i7. Not that I am saying thats the reason but possibly since I changed also to AT&T U-verse with a different Modem which is also used to run my TV programs.
Anyways, here's the issue.

When I get online with any game, it "jerks" at about 2 second intervals, and it will do this for about 5-10 mins of playing then it goes away. period. Not to start back again until I stop the game and start again later. Now, there has been a couple of times where after about 30-45 of playing it would return, if only for about 5 minutes then clear up. Similar to lag. It never did this with my previous AT&T Modem. Not until this larger one was installed for u-verse. I have had AT&T run a diagnosis on it, replace it, and check my lines inside and out and all is fine they say. Casual web browsing is fine as well as everything else. Only online gaming is doing this. Although it lasts , like I say only about 5-10 mins, its very annoying. That's an eternity when you first jump into a game like, lets say COD4 and you have this jerking that's interferring with your aim and well, everything. By the time it clears up my kill/die ratio is in the negative.
I heard it could be something running in the background related to networking but I dunno. I'm running an i7 920 with a GTX 470 so I don't lack the power .
Anyways, any ideas to help clear this up and I would be forever in debt.

Thanks