Sounds like an OS problem. The 8320 had similar issues when first released that later OS versions fixed. Although if it were the same problem you would think it would be released without that problem.
Was that other phone that worked a BB or a different brand? If it was a BB, you might have to wait for a new OS. If it was another brand, there's lots of things to try that may help. Lots of people (including me) run a second router behind their ISP provided router to make UMA more reliable. Mainly its being able to set the phone to a high priority so the connection is stable.