I'd say the first thing you need to do is a factory reset on the phone. It's a bit of a pain, but it'll happen if it has to go off for repair anyway. Back up your content first (photos, music etc), make sure your contacts are sync'd with an online service or export them and then back up the resulting file. If you want to keep SMS and call logs, there are apps which will do that, if not, don't worry. You may also want to make a note of your homescreen layouts for ease if replacing them later. A factory reset will put your phone back into its out-of-the-box state, so you'll be setting it up from scratch again and, with luck, this will fix the wifi issue.
The other possibility is that this is a memory issue. You could have a look under Settings|storage and see how much there is left. If it's looking tight, you may need to do a bit of deleting.
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