iPhone 4 battery charging mystery

[Patmanliness]

My written Finnish sucks... I will write this in English, feel free to reply in either suomeksi tai ruotsiksi. Ongelma koske mun svedu kielinen vaimon iPhone 4.

Wife's iPhone 4 (12/2010 malli) goes instantly from 92% battery life, to dead, nothing, nada, game over.
I plug it into the USB on the MBP, and get it rebooted. However, it won't charge.
Google, Google, Google.
I restore it through the backup, no luck.
I go hardcore, and try to restore it through iTunes - nothing but the message that, "There is a problem with your iPhone". Blah blah blah.
It is stuck in Recovery Mode after that.
She goes to the Apple Store, it takes 20min and the Genius has it back up and running, and it even charged up to 9%.
At home, I restore her data through the backup in her MBP, all is good, but it won't charge. Not through the USB on her machine, or from the wall socket with the cable.
Today, she goes back to the Apple Store: it charges fine from both their plug-in cable and through USB, so no problem. Or?
She then walks to work, plugs the iPhone in, and it won't charge.

Wtf? I use the same cables to charge my iPhone 4 and I have never had this problem.

Any ideas?

[egoste]

New charger & cable from the Apple store where it worked fine...