Not down to the Orange / t-mobile cell sharing deal, as the current arrangment is only around 2G services (although 3G services will follow soon-ish)
If you are connecting to 3G and its then dropping to GPRS or EDGE, the dongle / cell site cannot sustain a strong enough signal so they drop your network access to something more reliable.
In this case 2G (GPRS and EDGE)
3G cell sites breathe, the more users on a cell the smaller the coverage becomes, a year ago there may have been less users on the cell but you were on the edge of the coverage footprint.
More people over time have started to use 3G iphones etc.. and your service area has now shrunk more!
If you want to post or PM me your postcode, i'll take a look.
If you think it's something else you need to get another Orange handset / dongle in the same location to see if it shows the same issues.
If so then phone Orange and ask to open a network fault.