My Samsung Galaxy S keeps force closing applications!!?

i had the same sort of problem when i got mine. im on the 3rd replacement already. however so far so good. i found if i download to many apps my phone cant take it or if i use it continually(not really helpful because i got this phone so i could use it Between sixth form and home for work). mine didn't come with a memory card, but i bought one and just save my pictures/contacts to that. although i still loose everything on the phone every now and again because it 'forces upgrade', every time iv spoken to my network they say its the phones software and all the can do is replace it when it becomes impossible to use. i do wish id stuck to my google Gi though, had it for 3 year and it never let me down.