Problem with my Nike+ Ipod

Hey everyone, I seem to be having some sort of problem with my Nike+ Ipod when i've used it on my runs the past few days and I was wondering if anyone could help diagnose the problem.

I run on the treadmill for an hour at 7 mph. Therefore, when I'm done with my warmdown, it normally (or should) give me a reading of like 7.1/7.2 miles.

The past 3 days, however, the reading has been getting lower and lower. It was 6.85 after my warmdown, then yesterday 6.72, and now today I had to run an extra 3 miinutes just go get to 6.5. When I go to the Nike+ website to the log the runs, the graph of the little runner person is different than from what it used to be.

Before, my graph would like this: ------------------------- (which is right, because I would run at a constant pace).

Now, it starts off at a constant pace but then will randomly take sharp "valleys" almost to indicate that my speed drastically decreased when I never stopped or changed anything at all, like this: -----______------______------------------___.

What's going on? Is there a problem with my sensor? Should I attempt to recalibrate? Any help would be great.

Mine will do this too -- the graph thing. It doesn't bother me. Occasionally during a run the sensor may lose its touch for a few seconds, therefore appearing to slow your pace. Sometimes, mine will tell me I'm running a 20 minute pace, which obviously, is not true. I'll push the button again and the pace will be back to normal. If it's not affecting your overall average pace, I wouldn't worry about it too much. It's not going to be spot on perfect every single time.

I just ordered mine and some new shox. waiting on the shox to arrive to be able to try to run with the Nike+.... although i have read on the net about a lot of people having problems with the sensor picking up their pace....

Definitely recalibrate. I find this happens from time to time, and I just need to recalibrate. It will never be exact, but is pretty close, especially for the price. 

I recalibrated today and that seemed to do the trick, thanks for the advice, I didn't realize that calibration had to be like a monthly thing.

DON'T CALIBRATE ON A TREADMILL.  When you run on a treadmill your stride is totally different (the belt pulling your feet back, for instance), so it will not be possible to get both an accurate reading of your pace outside and and accurate reading of your pace on a treadmill.  The Nike+ help guide tells you not to calibrate on a treadmill.  Instead try a highschool track, or a 500+meter stretch that you are sure of the distance to get an accurate pace.  I had the same issue when I got mine.  Also be sure that the sensor is totally flat (if you don't have the shoes), any sort of tip from horizontal will screw with it too.

If you've had it for a long time and it constantly loses the sensor, it could be the battery.  They do die and you can by just the replacement sensor.

I unfortunately have no choice but to calibrate on a treadmill. I'm up at school and the only track that I could possibly use is as of this moment covered in a foot of snow. I calibrated it for the first time on a treadmill as well. The next day I ran outside on my typical route which according to mapmyrun.com and my car was a total of 6.1 miles and the ipod told me 6.23, so i'm not too worried about that. I realize it won't be spot on accurate; I was just concerned as to why every one of my workouts were subsequently getting shorter and shorter according to the iPod when my workouts were exactly the same.