I found the solution on another thread:
Premiere 5.5: The stutter effect with my computer is on both, 1/2 resolution and full resolution.
Premiere 5.0: The stutter effect is there on 1/2 resolution. But if I put it to full resolution it runs perfectly smooth. The CPU doesn’t have to work for the compression to put it to 1/2 resolution, thats why…
Why does it stutter then with full resolution on 5.5? No idea.
So I am back to 5.0!
Cheers