Sorry for adding so many comments in a short time - but this bug is just driving me nuts :-(
I did some test with the "noapic" and "acpi=noirq" options. None of them for itself solves the problem, and not even both together, as I just found out. It's important to disable the splash options too!
Not using the splash option makes the problem with the repeated characters much better (in this comment I had it just 3 times) - so somewhere the framebuffer (needed for the usplash) causes a problem.
Additionally using "noapic" and "acpi=noirq" nearly makes the problem go away - but if you have a dual core cpu, only one core will be utilized.
It would be great if others, who are experiencing this problems, could try out my suggestions and give feedback. Thanks!
Is this problem maybe related to a special kind of cpu? Mine is an Intel Core 2 Duo.