I have tried to take 180 thirty second exposures by setting the Interval Timer in the camera to 180 and setting 30 seconds on the shutter speed and the shutter locked in continuous mode but it only takes 100 exp. I have also tried it by not using the Interval Timer and locking the shutter on continuous and manually timing say one and a half hours of 30 second exposures,but same result it stops after 1 hour or 100 exp. I have tried on a D3000, D200 and the D7000.
I've got a D300 handy, so don't know exactly how this might translate to the D7000, but: in the shooting/display custom settings is a "max continuous release" option, which maxes out at 100. I suspect this is the limit you're hitting; certainly it's the explanation for the trouble with using the continuous mode.
When setting up the interval timer, have you stuck around to see how it works? I ask because my experience is that setting 30 second exposures to occur at 30 second intervals doesn't quite work. The exposure is 30 seconds + minor processing time + noise reduction (if enabled) = more than 30 seconds. The result is that a photo is taken every minute, not every 30 seconds.