Quicktime freezes every single time I'm finishing a recording
@fbara was right: It is the ram, but it is not obvious. Basically, what I am recording now requires a whole variety of software to be open at the same time, quickly reaching the limits of my Air's 4GB of RAM.
Once you use more than 70-80% of your ram, stuff starts getting compressed and swapped, and apparently, Quicktime cannot quite work with that.

Solution: Get more RAM and send complaint letters to Quicktime for not informing you why your recordings fail.