Problem with 3g losing connection

[spark001uk]

Ok, been scratching my head with this one. The problem is, open browser, start using it, the 3g/3.5g establishes a connection obviously. Then after browsing a few pages, the loading bar will just creep up, continuing to say 0kb, followed about a minute later with a no gateway reply message. To continue browsing, this is what i have to do- minimise and come out to the standby screen, press and hold end key (ends the packet data connection), then go back into the browser and carry on, then it will reconnect again and be fine for another few mins.
I've narrowed it down to what i am pretty sure is the phone itself, as i tried another network's sim in it and the same problem. Ive had the settings sent to the phone, i've done manual settings, i've changed location, i've changed the firmware, everything. Can't think what else it could be?

[spark001uk]

Ok i think i've got somewhere now - after eliminating many variables, i tried disabling HSDPA, forcing it to 3G, and so far the connection seems stable... but if so, why should it have been losing connection altogether if the HSDPA signal wasn't satisfactory, rather than just falling back to 3G??