It's Orange. It's not a case of switching networks though.
When resetting it to 3g and not getting any 3g (anywhere) after the initial switch to 2g I have removed the sim and tried it in another phone and got 3g straight off. This was to establish if there was a problem with the sim or not. Put it back in the SNS.... no 3g.
Did a factory restore and 3g came back instantly.
Tried the whole thing again to confirm the problem
(selected 2g, no data - reverted to 3g). 3g did not kick back in. Did factory restore and 3g kicked back in instantly.
The only time I've seen something similar is when I'd used the "Select a network operator" option and locked it to one particular network, so try putting it into non-2g-locked mode and use the "Network operators" option and hitting "Select automatically".
BTW have you tested battery use to see if disabling 3G in your situation really makes a difference? Given that you can't get 3G and it goes to 2G, does forcing it to 2G really make a difference? I've not tried it so don't know, I'm just asking if you've found a way to verify you're making a difference or just assuming it'll make a difference.