Boarduino and minor servo issue

So I have successfully interfaced a servo with a arduino and a pot, and can change the position of the servo with the pot. When the boarduino is hooked up to the PC with the FTDI cable, there is only a slight wiggle in the servo position. When I disconnect the FTDI cable, the wiggle becomes more prominent, like 15+ degrees. The wiggle has a frequency of about 0.5 hz.

The Boarduino is externally powered, and I am using a converted ATX powersupply as a benchtop supply.

I've tried a filter cap between the servo power and ground, but that doesn't seem to be the issue.

I assume somewhere, that I have a voltage that is floating?

Weird, I just noticed when I grab the metal base plate of the breadboard, the wiggle becomes more prominent even when the ftdi cable is plugged in. Perhaps my power supply ground isn't robust enough?

Or is it some sort of capacitance issue?

Make sure that is there a good electrical connection between the boarduino ground, the servo ground and the external power supply ground?

Its all a common ground.

OK, bizzare, I had the pot plugged into a0, when the script is looking at a2. Somehow it still worked, albeit with a wiggle.

When I plugged into A2, the wiggle went away.

Though my circuit seems to have some capacitative sensitivity. When I bring my hand near the pot, the servo starts wiggling slightly.

There is interaction between unconnected analog inputs, inputs that are floating will be influenced by an input that is being driven, perhaps that is what you were seeing

I figured that. Is it safe to pull them down to zero with a resistor?

Just make the unused pins... outputs.