Wifi access usually fine but internet connection unavailable in sites where it previously worked, how can I fix it?

[AC Question]

Wifi access usually fine but internet connection unavailable in sites where it previously worked

My HTC (Desire HD) works fine picking up wifi everywhere it needs to. However in some saved locations it connects to the wifi immediately but I cannot access internet. The error message says "page unavailable ... check wifi connection". I don't understand why this happens to be able to fix it. I have tried clearing data and deleting the wifi list but results the same after reconnecting.