Well I was switching between 2G 3G and 3.5G here last night.
Thing is, this is the first device that has even SEEN 3G around here. So I think the N900 is super sensitive which is why it switches between more often, because it can get a connection even when its fairly poor but that naturally means it will drop more, because someone walking about in their house, trees blowing, anything basically will alter how the signal reaches you and disrupt it.
WiFi is not bad either on the N900 except I find with a weak signal ultimately WiFi stops working and requires a reboot. It seems to crash the WiFi chipset. But at the end of the day, you shouldn't be using a weak WiFi signal anyway as it will be unpredictable on any device.
But at the end of the day, you shouldn't be using a weak WiFi signal anyway as it will be unpredictable on any device.