When I power it through a USB port (tried 2 different computers), it works like a charm. I then tried switching over to powering it with a 5v usb wall wart rated for 700mA and the servo wouldn't move. Then tried a 2A wall wart, and it moved for a second, then no response. Finally tried a 2.3A 5v mac wall wart, and the servo started jittering. It was doing something similar to what I programmed it to do, but very unresponsive.
I know it's a big servo, but there's no way it draws over 2.3A under no load. I'm also thinking this is not the issue as USBs only put out 500mA AFAIK. Unfortunately, I don't have a multimeter to diagnose the problem, or a higher rated 5v power supply. Has anyone else had this problem with wall warts? Is it that they just don't put out what they claim, or is it something I might be doing?
The wires are fine, but ahh, I never connected the grounds. When I was powering both the arduino and the servo with the PC, I guess the grounds were connected internally to the CPU. Now that i'm using two different sources, I have to connect them externally? Just by my (limited) knowledge of electronics, I don't see why it would matter, but thanks. I'll try it now and report back.
I'm guessing the differing performance depending on the wall wart was just a fluke.
Woohoo, it's working! Thanks for the answer and the explanation. It even worked with the 700mA wall wart. Not sure why I got different results using different wall warts, but now that it's working, I don't really care. Thanks for the help!