Voltage is not your problem for jerky servo movements, it's the amps they are trying to draw. The 1.5amp power supply will not be enough to power servos for when they begin to move, as they will draw a few amps. Read the tutorial linked below, that explans power supply requirements for powering a servo and multiple servos.

https://synthiam.com/Tutorials/UserTutorials/163/1

You are better off with a LiPo or NiMH battery pack or switching power supply with a much higher current output.

As for the servos, what are the specs for the servos you are using... mainly, what is the voltage range for the servos? If they are rated for 7 volts, giving them 9v could quite possibly damage them. Only use the correct voltage range that the servos are rated for, no more, no less. The EZ-B can take up to 16 volts, but remember whatever voltage you give the EZ-B, will come out of the red power pins on the digital ports.

The link I supplied above, will also give you the details of the EZ-B's power specifications.

Read this.... Understanding volts vs amps

EDIT Disregard... Steve already posted the link...

I find it really strange that volts/amps correlation confuses so many people... And I am not talking average people but people who are advanced programmers and IT gurus.... confused You should know this not because of just robotics, but because all electronic devices at work and in your house/car etc... I mean you can't install a simple GPS or cell phone outlet on a motorcycle without knowing what is the difference between amps and volts.... These power questions seem to come up frequently on the forum.... Just sayn'

@Richard.

I find that too. Also, the people I speak to, friends, family, old work collages etc, don't seem to see amperage as important as voltage, or disregard current altogether. Like I say to them... It's not the volts that will Kill you, it's the Amps.

That's why many people survive lightning strikes.... Millions of volts but very little current behind it... It may be that even a 12V car battery has more current than a lightning bolt? Who knows LOL

I learned this lesson the hard way when I was 17 working in an arcade fixing machines. Lots of amps in the back of a CRT type monitor or in the coils that make the flippers and bumpers work on a pinball machine. I was thrown back and put a hole the size of my 6'4" 240 lb frame in a wall one day while working on a machine without grounding the screw driver that I used to pop the plug on the back of the cable carrying power to the back of a CRT monitor. It taught me well that day...

I still laugh about this with friends that I know from that time. I am sure I smelt a bit burnt, but the lesson was "pounded into my head" so to speak.

@David... It's funny now right? I did something very stupid too... many years back.... I was testing a spark plug on my motorcycle at the time. I pulled the plug so I could see if it was sparking (because the bike wouldn't start). I put the plug back in the spark plug cable cap with the intention of grounding it against the engine case. Then pushing the starter I would be able to see if the plug would fire or not. Well I was holding the plug in my hand while I pushed the starter... I must of also grounded myself to the frame because I got a nice dose of ~30,000v from the bikes coils... Rattled my teeth... Very stupid lesson learned...

Yea, I have made the mistake of holding spark plug wires also. You know, it is really amazing that we survived our youth. I used to work on all of my own stuff to fix it because I had no money to spend to have others fix things. I used to get so mad at my dad for making me spend the weekend fixing my own car. I remember thinking that when I get older, I am going to make enough money to pay for someone to do this crap. Now that I am older and have the money to pay someone, I enjoy fixing my own stuff. My children, on the other hand don't yet have the appreciation of knowing what it feels like to fix your own things. My wife and I both would rather remodel our own kitchen than pay someone to do it.

How times have changed, and how smart my father is...

@David... I had a very little supervision as a child .... Yes, it is surprising my brothers and I ever made it past our teens....

I have always fixed my own stuff as well... To save money but also to learn how stuff worked.... We didn't have the luxury of youtube as kids either. So if anything kids now days should be way better at repairing and building their own things...