Eisenhans , I have nothing to do with the thing you are describing. You can check the capx your self and see everything under the hood. The fact is that for some reason full-screen is more stable. Also, the fluidity on the lower frame rates is perhaps related to the optimizations that Ashley did on the r198 build.
When you say that it gets fixed at 30 fps at start up, you mean that if you leave it running forever, it will still stay at 30 fps and never go up? On my system, each time I run a preview there is a few second period (around 10-15) that there is great inconsistency in the frame rate, after that time period the game catches up and stays more or less stable. Especially with this latest beta build.