Its doing this because you are running low on onboard memory or memory that you use for applications, for example my 9300 came out with OS5 and I upgraded to OS6, before the upgrade I had something like 120mb free onboard(you can check if you go to memory details in options or on the top right when you are in myworld of app world), now after the upgrade my free memory dropped to around 60mb meaning that when I browsed it used this 60mb quicker and resulted in either "page to large closing page" or another error saying I must close some tabs.
Bottom line is you need to free some space up by deleting apps u don't use, you can also use a program bb boss(at your own risk) it will allow you to delete languages and the "locked" default pictures and ringtones.