Firmware 4.37 for the 7650 is set to be released and available from Nokia Service Centres on or around December 22nd (but may be available before this). The new version fixes several bugs including the drop of line2 after 59 seconds bugs (that only some people have had).

Firmware can only be updated by Nokia Service Centre (a special machine / software combination is used).
You can find out your current firmware by typing the following at the main screen: *#0000#.