Take a look at your drive light and see if the "jitter" is happening when the drive is in use. This is what I noticed when I tried a Toastscan years ago. Whenever the drive was in use, the screen dimmed a bit causing a flicker.
I think the Toastscan uses so much power that when a drive is in use, the screen dims. (but no, I didn't do a "power usage test" to see if this is actually the case - just my own theory)
"It's the engines captain, they canno' take it!"
