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I've found that orange mobile internet can be very patchy at times when I'm out and about. Sometimes the 3g network is basically unusable. Other times it is fine.
My home WiFi with BT is rock solid and the Desire works very reliably with it.
Do you know the quality of your WiFi signal? There are apps that can show you the signal strength. Does your WiFi work reliably with other devices? Does a rock solid site like Google give many timeouts?
If you can rule out these problems I would try a factory reset to make sure that your software is in a known state. If you have no joy after that, it's time to get a replacement phone.