Had a search and and not find a similar issue

When at home I use wifi to connect and have dual mode selected in Settings>Phone>Network and signal varies between 2g/3g however when I get to work (a 20 minute drive that has 2 and 3g cells) I find I have no signal and when I check Settings>Phone>Network it has changed to 3g only. I tend to leave IM on most of the time and use autodisconnect for those very rare occasions when I go offline

If i remember correctly, i think this is a AutoDisconnect issue.

Personally, i just use the 2g-3g-dual applet to switch between connection types, rather than use AutoDisconnect because it never uses Dual mode, only 2G and 3G.