HTC One M7 :: WiFi Connection Timeout - Only Work For 5 Minutes

I recently purchased a HTC one (4 days ago) and already exchanged the phone once do to a defect in the wifi & now I'm about to change the phone again due to the same issue.

The phone will connect to the internet via wifi, but only work for 5 minutes, then nothing. The phone shows that a wifi signal is still available & that it is connected, but if you try to surf the net or refesh emails/facebook/weather, nothing. A message then appears on the phone stating connection timeout and you have to turn wifi off, then on again, for another 5 mins connection.

So far I've tried..................

1.Different routers, at home/mobile broadband/work/friends - same issue.
2. Contacting HTC and using 'safe mode' whilst they talked to me - same issue.
3. Factory reset - same issue.
4. New phone, changed at a recognised shop - same issue.

Luckily enough I'm within my 7 day phone exchange with my phone provider, so the things going back & exchanging for a different brand...............

On a different note, my wife also got a new phone at the same time, HTC One Mini & this is working ideal.