Markus Robert Kessler wrote:
> Upper right on the screen there appears a yellow flash all the time and
> one of the boot messages reads something like "undervoltage detected".

Just believe what it says. You need either a:
- power supply with higher voltage
- better quality power supply with more constant voltage
- shorter usb power wire
- thicker usb power wire

Or all of them. Check it on the command line: "vcgencmd get_throttled".
It'll reply like: "throttled=0x50005". The first 5 means it has at one
point during this run been throttled due to undervoltage, the second 5
means it is currently being throttled due to undervoltage (a different
reason could be overheating). So sort it out or you will get lousy
performance.

bit 0: undervoltage (voltage < 4.63 V)
bit 1: capped (temp > 80 C)
bit 2: throttled (temp > 85 C or undervoltage)

From
https://www.raspberrypi.org/forums/viewtopic.php?f=63&t=147781&start=50#p972790
: "Throttling removes turbo mode, which reduces core voltage, and sets arm
and gpu frequencies to non-turbo value. Capping just limits the arm
frequency (somewhere between 600MHz and 1200MHz) to try to avoid
throttling. If you are throttled and not under-voltage then you can assume
over-temperature. (confirm with vcgencmd measure_temp)."

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