The users of Nokia N81 may be knowing about this issue. The issue is with phone memory and is there irrespective of how much free memory you have in your memory card. While there appears to be an inherent problem with this handset, you can remove some programs/applications from the phone memory that are not absolutely must for you to fix this problem. To do so, just go to, "Menu" > "Applications" > "App. mgr." and uninstall the programs that are not absolutely must for you. Before uninstalling, you can check the memory that a program is using from, "Options" > "View details" and then decide if it would help to remove that. Any new program that you have installed in phone memory like, "Opera Mini" for example should be removed from there and should be installed in memory card.
Additionally, it may be a good idea to clean cache from the default browser frequently.