ggpspeedtest

I did a Manual test and tried using 200MB. The download speed averages around 10 - 12 Mbps but then it suddenly jump to around 45Mbps (see screenshot) which is a surprised since I should only be getting 18Mbps. I think I may have a problem with the line or it could be due to some kind of traffic shaping or QOS on AT&T's side. AT&T uses speedtest.net and as far as they are concern there's nothing wrong with my connection.

My issue with video streaming started like a few weeks ago. I've been using speedtest.net to check my download speed and the results are always between 18 and 21 Mbps with occasional slower speeds of like 15 Mbps but I still can't watch TV shows or streaming videos since they are choppy. Then I started testing my speed using testmy.net and I get a totally different result from what I'm getting from speedtest.net. I attached one of the results below and you would notice that my download speed varies during the duration of the download test. Does someone have an idea what could be the issue here? Is it my PC or is it U-Verse? Thanks!