Do they always climb? If they are both doing that, and it's not all the time, it must be something to do with the transmitter, your use of it, or outside interference. It would be too much of a coincidence if both the planes radios were acting in the same way as each other. or it's being caused by balance or anything else in the planes themselves

Are you aware of anything that accompanies it like trains going past or what you ate for breakfast?

David C.