I have found that the HSDPA/3G connection is not very reliable. I often find myself disabling and enabling "Mobile Data" because the device tells me I have an HSDPA connection but all apps/browsers consistently fail to connect. Either because the connection broke or it could not connect to the proxy (sic). And if my connection deteriorates from HSDPA to 3G, Edge or GPRS it does not make a difference. Once it fails, it will not recover.
Disabling and enabling sometimes helps. I have to say that I use the device mostly on the train and this is suboptimal at best, but my HTC Desire S nor my Nokia E71 had this issue so I do suspect the Google Nexus phone or this firmware (Jelly Bean 4.1.1) :-(