I have a Gh2 I bought 4 years ago and then purchased a second Gh2 (new) 6 months ago so I could film using two cameras. I ran into problems with both cameras last week. I was filming outside in the shadows in cinema 24p in manual mode. ISO was 400, aperture at 12 and shutter speed at 50. I white balanced to the sun mode and then later manual white balanced using a grey card. I found that both cameras where shifting exposure and color. Background became lighter and actors face which was well color balanced to good skin tone slowly turned red. This shift took place over the first 15 seconds of recording. If anyone has any idea how to set camera settings to avoid this kind of drifting, which is making the scene shot unusable, would be greatly appreciated.
I don't know if this helps but I noticed that even the slightest change in cloud density will affect white balance. It's pretty sensitive. On another note, you sound like you know what you're doing, but is it possible some setting is not set to manual? Are you in shutter or aperture priority?