If you don't work on an actual mechanized assembly line, why act like you do? I don't think the average human is satisfied by mechanistic repetition without variation. All the ergonomic experts come around telling us to vary our physical positions to keep our limbs from becoming unduly stressed, but I think you need to vary your mental position just as much.
Shift your tasks based on your energy and the tools at hand.
I know that I tend to have an energy slump in the early afternoon, so that's when I schedule meetings or routine tasks. In the late morning and late afternoon tends to be when my brain is ticking over at high speed, so that's when I like to do heavier mental lifting such as writing that's not based on previous work or in-depth testing of a complex problem.
Try microbreaks and nanobreaks.
When my work could be very stressful due to more falling onto my plate than it can hold, I can often fend off that overwhelmed feeling by taking a moment to remind myself of the good things in my life. Between tasks I'll take a quick look at one of a few sites I check every day – three friends' photoblog sites, a web-based comic, and, for the longer breaks, a link-blogger who frequently has tech news of interest to me at work, but who is so interesting as to be a risk of longer distraction, and a snarky news commentator who does a daily video post. Pulling myself out of context when I feel the stress building diffuses the tension and allows me to return to work a few seconds or minutes later with a fresh mind.
Nanobreaks are a new enhancement to my stress-busting repetoire. I have stocked my screen saver and changing desktop pattern with pictures that make me happy: friends, flowers, landscapes, and other favorites, mostly copied from my Flickr contacts. When I need a little jolt of happiness, I just minimize my windows (F11 on the Mac, yay!) and there's something on my desktop to bring a smile to my face. A second or two may not seem like it would make a difference, but for me it is profoundly useful. I recommend any cube-dweller who doesn't have a beautiful view to use for recharging give this a try.