I have a couple of suggestion that might help with your problem. First, there is a smoothing function for pace. It would be under Setting>>Running>>Speed Unit. You can then play around with the smoothing. I have mine set on most and it still bounces around some. What I do, that I think is even better, is setup an autolap and use average pace instead of pace. Depending on your speed and the accuracy you need I would set the autolap for .5 mi or 1.0 miles. Right now, I have mine set for .25 miles but I've been doing things there I want .25mi intervals. Lap pace would be your average pace over that short distance and stays fairly constant. The trouble is GPS is only accurate to within a few feet. So the distance between 2 samples can vary wildly but over time, those inaccuracies tend to cancel out.
As far as keeping a constistant pace, you can try setting pace alerts. You can set both a minimum and maximum pace. I would put a little bit of spread between them to allow for natural fluxuation in running speed and inaccuracies. I use it primarily to keep me from going too fast, and If I want to stay below a 12:00 mile I'll set it from 11:30. When I would set it for 12:00 I would average less than that because it would beep and I would over compensate.
Also, you might want to download and checkout a piece of software called
SportTracks. Depending on your needs, they have a free version and a pay version of the current version. You can also find the
previous version that was freeware. It allows you to do much more analysis than the Garmin Training Center Software. That being said, GTC is very nice for setiing up Advanced workout. I use it to setup workouts because I use a Galloway style walk/run. I can also put them on the calendar and upload them to the watch for easy finding.
I really like my watch and have found it useful in achieving my goals.