mischl, You will need at least 6.2 volts dc at the input of the 7805 for it to regulate at 5.0 volts on the output. A typical 7805 regulator has a 1.2 volt drop internal. Therefore if the input voltage drops below 6.2 volts the output will drop to the input voltage-1.2 volts.

Dave Purola,
N8NTA