shimmering generally points to an issue with the refresh rate, doesn't it? at least, if it's not the cable.
wait, 21.5" monitor, and it's a 1920x1080 res? which one is that? i thought that you had to go past 22" to get a monitor with full hd resolution standard. if you're forcing it beyond standard res (probably 1680x1050, like my 22"), then the shimmering could be the monitor trying to update pixels that aren't there, or trying to keep up with a high refresh rate when it's not built for that res.
phill, he's using a lappy with a secondary monitor attached, so there's only one cable.