Extremely high ping every minute

Hi.

I've been using U-verse internet for almost 2 years.

My internet speed is fine. It's not slow. It's about 20 mbps download speed which I paid for.

But my problem is about the ping.

I had no problem until now but since 2 weeks ago, I am having this annoying problem.

I play some online games 'League of Legends' 'Starcraft 2' and 'Diablo 3'

I don't recognize this problem when I just surf on the internet.

But when I play game, the ping periodically goes high over 900ms.

Sometimes it stays at 900 ping for a long time so I can't play the game at all.

Sometime it keeps going up and down over and over again. Can't play as well.

What makes me mad is that most AT&T live chat representatives don't know what I am talking about. They keep asking me about the speed even though I tell them the speed is fine.

I just feel like they don't know about the ping at all.

They sent me a technician and changed the modem. Didn't fix. Actually this made the ping higher than before. It was about 500ms before the guy changed my modem. Now it's 900ms.

This started happening after AT&T changed my internet channel.

My internet speed was about 10mbps even though my internet plan supposed to give me 20 mbps. So I asked them to fix the problem so they changed the line (or channel, I don't know the exact name). After that, this ping problem came up.

please see these two links of speed test.

http://www.speedtest.net/my-result/4249546005

http://www.speedtest.net/my-result/4247436293

AT&T Live chat representitives keep asking me to do the speed test which is so helpless.

But when I do the test, the result has no problem. So I did the test when the ping went up high.

This result is not the permanent status. Everything comes back to normal after a minute.

and then this happens again after a minute.

Tried

-Rebooted the modem many times.

-The repair guy came and changed the modem.

-Asked them to change the channel again

-Ran the vaccine program.

-Changed the outlet that the modem is connected.

Nothing worked.

Please help me out. THIS IS MAKING ME CRAZY

FYI

1. I am using ethernet (wired) internet. This is NOT wireless.

2. I do have an additional device, of course. I have a desktop which is connected to wired internet. and I have an additional laptop and a phone. total three devices.

But please don't say that this happens because of the additional devices because I DONT USE THEM TOGETHER AT THE SAME TIME. This ping problem happens even though my other devices are TURNED OFF.