Finally, I've found a workaround. I splitted my huge project into smaller parts. The parts render to Prores422 files fine without a crash. OK.... it takes up to 12 hours for some parts, but I don't care. In the main project I use the Prores422 files and added framing behaviours to the camera. The project is not looking exactly as it was intended in the beginning, but now I can edit without waiting a long time after each mouse-click.
Eventually the 27" iMac. 2,8 GHz Quad-Core Intel Core i7 , 8GB RAM, ATI Radeon HD 4850 arrived. (btw: no Flicker, no yellow, no HDD noise, no dead pixels - just perfect in every aspect.)
After 3 days of working with this power-horse, I am happy with Motion now. There is no more need to save work after each keystroke. Rarely there is a beach ball and when it appears then it is only temporary and not the sign of a incoming crash. The project files that used to take 11,4 hours to render take only 93 Minutes now. I dont have to wait after each mouse click until the machine recovers from the input and can work fluently.
To draw a conclusion: Don't expect a 24" iMac with 4GB and GT130 graphics to work well with motion. For smallest projects like lower thirds, credits or titles it might be ok. When it comes to more challenging projects switch to more powerful hardware.