Support data issues using UK GSII in America??

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http://androidforums.com – Hi All, I have returned to American from studying in the UK for a year. I brought my I9100 with me, and I purchased it in the UK on the Vodafone network. Now that I am back, I went to AT&T to get a SIM and start using my old saved number. They let me try putting one of their SIM's in my phone before I purchased, and it locked onto H+ data. Great I thought! I loaded up a youtube video and it downloaded excellent. However, now that I am out and about, the phone seems to act up a lot. It will sometimes get into this switching loop where I can watch the data switching from 3G to H (General)