After many tries with frame limiting and various other stuff (deactivating multi GPU, changing GPU settings away from 'let game decide' and back to it), I read some crash logs from users on this forum. Some contained warnings about other programs using the GPU, which led to some kind of conflict.
Therefore, I decided to deactivate windows' aero desktop (it uses the GPU and its 3D capabilities), and did not experience a single freeze since then. What I get, however, is a screen flickering just between matchmaking load loop and the real loading screen with the three bars, but I can live with that.
If you experience freezes, try deactivating aero. It helped for me. Also deactivate Multi GPU in NVidia control panel (or ATI's equivalent).
Intel i7 3770K @ 4,6 GHz
16 GB RAM
NVidia GTX 690, 4 GB
Win 7 Ultimate x64
OCZ Vertex 2 160GB (System disk, 100GB free), Seagate Barracuda 7200.14 3TB (Game disk)
Edited by assiprinz, 28 February 2013 - 12:24 AM.