iphone 4-text notification problems

Hello
Im having some problems with my iphone 4 with ios 5.1.1. Sometimes when i receive a text it just won't let me know by ringing/vibrating or leaving any kind of notification on the screen, the text will just be added to the conversation window and sometimes i only notice it when i go back to that conversation window.
Im using the stock text app and this happens a bit randomly, so sometimes it works fine and all of the sudden there will be one text that won't be notified.

Other problem i have and it also involves the text stock app is on the phonebook when i try to send a contact info/number using the "share contact"-"by text" option, the app will switch to the messages app but it will crash and so i can't automatically send the number.

Does anyone know any solution for these problems?

Thanks!

 

Thanks for the reply and help.
The text notification problem does happen when im texting a lot, so it might be that and i will pay more attention to see if thats the case.

The contact sharing problem, i did the soft reset but unfortunately the problem continues :/