Quote Originally Posted by koossa
...If I connect my Pot's left pin to +5v and right pin to GND and Vo to middle pin and measure the voltage over my 7805's input voltage and gound it measure 4.3 volts and if I remove the pot it give me 8.03 volts.
Well, in this case you should check your power supply.
If the Input of the 7805 really drops that much with a resistive load of several k there is something wrong with your PSU.