It is caused by the low memory available in your phone.
Check and close active applications not necessary at this time. You can long press the Menu key to see the list of active applications, and then select the applications which are not needed and close them.
Some tips for using the browser:
- Try to avoid open web pages with big pictures.
- Disable graphics to save memory and speed up downloading Select Menu > Internet > Web > Options > Settings > Page > Load content > Text only.