my cell phone says memory full. help me

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i have a nokia n70 and this has a big memory card. and this is working well. but there are many messages. as all the messages are being saved in memory card there is enough space to save many. but still when there is a space of more than 100 mb left in card and also in phone memory 80% memory is free including the ram,it says some time memory full while messaging some one. and some time not. what should i do.