iPhone 3g - not visible in iTunes, not showing charging but charging

Hi!
That's my first post on that forum so hello everybody!
My problem is like that.
I had an iPhone 3g with broken power button. I was using it from time to time.
I installed whited00r on it and left on the desk. After few weeks I tried to start it as usual by plugging charger - without result.
After plugging to computer also nothing happened.
I decided to change flex to put phone in DFU mode.
I change the flex and I was able to rest the phone. Now it is normally working on Whited00r but computer still not recognize it.
What's more after plugging the phone to charger nothing happens. Battery status is not changing at all but after powering off and back on I can see that battery is charging slowly.
When phone is off and I am plugging the charger to apple logo appears and phone starts but claims no charging...
Do you have any idea where the problem is? If it is motherboard than OK, phone is gone. But if it is only dock I could easily replace it, but is it dock? The phone is turning on after connection so dock is somehow working...
How do you think? Is it motherboard or dock?

And one more important thing. In DFU mode computer also not detect my iPhone.