Temp sensing problems

Here's a little problem I'm having with the PID. My temp spikes every time the Arduino turns on the digital pin connected to the SSR (to turn on the heater). Here's what the graph looks like:

The measured temp goes up a couple of degrees when the Arduino is driving a digital pin attached to the SSR. Obviously, the actual temp isn't doing this. You can see that the duration of the error corresponds to the duty cycle - the upper part of the graph is wider when the SSR is on for a longer time, and thins out as the temp gets closer to the setpoint. I had thought this might be caused by the voltage dropping because of the heater sucking so much power (800 watts?) but it does the same thing when Silvia is powered off and unplugged. If I disconnect the (-) connection to the SSR, the error increases, but when I remove the (+) side, it goes away. Another strange error is that when my MacBook is plugged in to the AC adapter, the overall temp measurement goes up about five degrees. (I normally don't drag my adapter into the kitchen.) I've tried powering the Arduino from my adapter inside Silvia, an external adapter, an external adapter on a different circuit, and from the Mac's usb, all with the same result.

Because of the way the temp goes up when the Mac is plugged in, I wonder if this only happens when attached to the Mac. But I can't check this since I wouldn't be able to see the temp without the Mac. I may try to hook up my LCD display so I can monitor the temp without connecting to the Mac.