That's definately a frame rate issue. The Roundabout (Aldstadt Spandau; Spandau Old Town) is one of the busiest areas of the map, it's close to the Rathaus but also has a lot of traffic due to the nature of the roundamout, and all the converging traffic.
Although I don't get bouncing buses, the roundabout is where my frames do drop down a little.
What Graphics card do you have, and what Processor, OS ?
I believe some people using stock Intel Graphics Chipsets are getting the bouncing. For a sim of this fidelity, you really need a semi-decent graphics card. I got a 8800 GTS 512Mb for £50 delivered and it runs OMSI beautifully, so you don't need to spend a fortune.
But stock 64Mb GPUs aren't going to give you a good driving experience.
Cheers,