Xperia Mini/pro :: Don't Receive All Text Messages Sent To It

Since a few weeks now, i don't receive all text messages that are sent to my phone by contacts anymore.I can't pin an exact number how much messages are 'gone missing', but for example, yesterday i was texting the whole day with a friend,and of the on average 25 messages she sent, 3 never reached my phone. Also, sometimes when i get a message the text stops in midway and it ends with a piece of text from another message, that i received weeks ago. However, this problem has dimished over the past weeks. By the way, i have to add, i'm thinking of doing a factory reset, but for that i need to save all my contacts. i tried exporting all my contacts to my sim (using the import/export function in contacts),but when i used my sim on an older phone, only half of my contacts were transferred.I have to add that i got a notification that the sim was full, so some data wasn't transferred.