I had the problem, too.
Upgraded my iPad 3G 32GB to 4.2GM, lost WiFi every few seconds after starting a browser (didn't matter which one). Did a network settings reset. Didn't help.
Did a full restore without restoring from backup and had the problem immediately after setting up the WLAN connection. No apps had been installed at that time, and no email accounts set up. Didn't help.
Restored again, but this time restoring the previous backup. Didn't help.
Turned off MobileMe Bookmark and Note synching as recommended in a previous post - and the problem went away.
As described above, I also had the problem with the clean install, where I hadn't set up MobileMe yet. So I don't see why turning off parts of MobileMe should have helped, but it did.
When I had the problem, I tried...
- an unencrypted public WLAN
- my encrypted WPA2 private WLAN
- an Enterprise WPA2 WLAN
...and I experienced the issue either way. So the issue doesn't seem to be WLAN type / encryption or router related.