HTC One V really slow

I bought my HTC One V exactly one year ago, at first everything went really smoothly and it worked like a charm. It didn't take long before the phone crashed from an unexpected error. The phone spent most of the time in my pocket then, I would notice it because it vibrates then or the next time there would be that error-sign in the upper left corner. I started living with the fact it sometimes shut itself down, but now the messaging app is beginning to lag: When I want to send a message (or make a call), it happens that I cannot see my contacts. (which are imported from Google) But it could also be that I have to wait a minute or more because the messages for that contact are loading. How can I fix these problems with my phone? My Android version is 4.0.3