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I have had wifi problems on my new I Phone 4 since the 24th June at home with connecting to my router and get slow speeds and web pages that did not load and would freeze but have tried a little trick after seaching all over the web this weekend that seemed to have solved my wifi woes as of the last 15 hours I have had lighting wifi speeds and great connection so it might work for other users too.
1. Upgrade to latest firmware for your router.
2. Within router settings my mode was setup to use 802.11B & 802.11G mode, change this from either auto or B&G and select it to use 802.11B ONLY or B mode only.
3. Save settings, reboot router and reconnect with your I Phone.
This has worked for me after 2 months of woes. Hope It Helps
I Phone iPhone OS 3.1.3