Keyboard and touchpad sometimes freeze

I installed Ubuntu on my Macbook but now I encounter some problems with the Apple Laptop Keyboard and touchpad. They sometimes just stop working, not necessarily at the same time. After a few seconds they work again. The system doesn't freeze. This doesn't happen when I freshly boot into Ubuntu. I found at that when the mouse freezes and I hit a keyboard button, the mouse sometimes works again. What always works is the power button: when everything freezes, I hit it and I gain the control over my mouse and keyboard. Sometimes I have to press it more than once.

My question: How can I find out where the problem is located and fix it?

Thanks for any help.