As my Pi (from RS) never ran more than 2 minutes before freezing I did some research and came across the power supply topic.
I started measuring the voltage (in idle mode, no cpu load) between TP1-TP2 and the voltage at the USB-plug
on a bare Pi then with a Keyboard then with Mouse and Keyboard and then with everything (hdmi+network) connected
In short: The power supply always delivered 5.04 V
BUT the measurement TP1-TP2 fell from 4.9 V (bare) down to 4.8 V
and at that level (no cpu load) my Pi becomes unstable at ANY activity (like access to the sd-card, etc.)
looking at the schematic (thanks for supplying, this helped a lot) I found the missing 200 mV across the polyfuse F3
this brings me to my question: Is the Polyfuse out of specs or is this an expected Voltage drop?
If yes, the 5 V are not enough for a stable operation.