That don't sound right! Have you tried turning it off, taking battery out and then restarting it to see if that clears the problem.
If not you may need to run a repair via either PC Companion or SEUS. If you choose to go this way please make sure you have backed up your settings, copied contacts to SD or synced them (or both). The phone will basically be factory reset so you will be starting from a clean slate, which will hopefully remove your problem, but does mean that you have to reinstall your apps and setup the phone from scratch.
The other thought that just occurred to me is do you have a task killer running that may be killing the wifi or have you downloaded any apps that control what the wifi does? If you have uninstall them and see if that clears up the problem. If not you are back to my first two suggestions.
Maybe someone else can come up with more ideas.