Hi Joergen
Have a look at this thread
K5 generic battery
Even though it deals with third party batteries it is relevant to this problem. In similar threads here I have argued the point that even Brand name batteries (that are much more expensive) are often made by the same manufacturer in China and can display the same shortcomings known to generic batteries. In my Motorola mobile phone for example I run the original Motorola branded battery and a generic battery (bought on e-bay.) Both batteries are made in China.
When I run Sat-Nav on the phone where the power requirement is heavy on the battery the original Motorola battery displays the same characteristic as your Camera Battery, namely it collapses after a while and when I turn the phone off for a minute or so it comes up as full again. The generic battery does not have the same problem.
I really do think that your problem is battery related more than anything else. (I could be wrong though as I have no way of testing it.)
Greetings