Hi Allan,
Had this problem with my 3G. Hardware. I had to reflow the PCB. Rather than using a heat gun I used my oven.
My alternative was paying Apple to fix the motherboard. So the reflow, although a risk, was a simple decision. No matter what I was looking at spending the replacement cost of my phone to fix the problem. I did this instead
http://www.iphonetiquicia.com/repair-iphone-3g-wifi-oven/
I don't know if you are familiar with reflowing circuit boards so the tutorial may look crazy. But it's really just a less expensive tool (home oven) than the reflow ovens factories buy. Key is time and stable temp (use a second thermometer to be sure about temp).
Once I did the reflow not only did wifi work but my phone operating temp was cool rather than hot as it was previously. In other words, the reflow reduced the resistance in my solder joints that was previously caused by the micro-fissures. Saved me ~$200.