6230 GPRS disconnects

I previously owned a Nokia 6230 which I was able to use to connect to
the internet from a laptop via bluetooth and GPRS with. This worked
flawlessly, but unfortunately that phone got stolen.

I've replaced it with the same model phone, but the new phone seems to
disconnect the GPRS connection after only a couple of minutes usage.

The only difference is that the old phone had firmware v3.15 and the
new one has v4.43.

Has anyone else using this version of firmware seen the same problem?
WAP browsing via GPRS seems to work fine.

Chris