Flash box is used for Java based phones and is available in the market and is costlyhttp://europe.nokia.com/support/download-software/device-software-update
from the above link you can mannually download and install the firmware updates.
OrDownload and install NOKIA SOFTWARE UPDATER which is also provided in the above link and follow instructions in it and is simple to use.
The above procedures are done cautious and is better to done by an expert otherwise a simple mistake will make your phone dead
Your problem is automatic switch offsolutions------------
1.Free atleast 3MB of phone memory2.If you are using 512MB card ,it may be a problem.Try to use 1GB card.3.Do not overload the phone memory and memory card with lot of items4.Try to keep only 5 folders in main window of memory card and move other items to these 5 folders.5.Try retore to factory settings (menu>settings>restore factory settings)6.The restarting is due to virtual memory overloading and the phones virtual memory is shared from phone memory itself.So keep 3MB of phone memory free and if you are able to free more it will increase overall performance of phone7.Do not apply heavy themes.Use only themes of size not exceeding 25KB.The default nokia theme is good for phones performance