Question Wildfire turns off without reason

Hallo:

I bought my girlfriend a HTC wildfire this christmas. So far everything is working excellent. However, last week she came home telling that her phone was broke. It had been laying in her locker at work and when she pulled it out of there, it was turned off and it did no longer respond to the on/off button (the battery was completely full)

I pulled off the back lid, removed the battery and inserted it again. When I pushed the on/off button, it worked again (this is how I know the battery was full, the indicator indicated full capacity). This has happened several times again this week. The device is running stock froyo, is protected by a rubber case and has never been dropped. Any ideas?

Definetely I'm going to the store to complain about this and I'll try to get a replacement.