Sanalece,
That is a particularly poor way to jack up the output of an LM7805. The current that flows out of the GND pin is a stong function of temperature, the input voltage, the phase of the moon, etc.
The better way is to use a LM317 with a voltage divider (two resistors) to set the output voltage. If you have to use 7805, then the voltage divider upper resistor must be like 100Ω so that the current out of the GND pin is a small fraction of the total current that flows down the divider...