Generating countdown with timecode

I have been trying to generate a countdown from 30:00 to 0:00 using the text generater effect. I go to text and hit the generate tab, then select timecode. In the start minute window I put 30 and for interpolation I select deceleration. The numbers in the timeline start to decrease as they should but then they start to climb up and down in a somewhat random fasion never reaching 0:00 at the end. Any ideas why the countdown doesn't go from 30:00 to 0:00 with this set up? Is there another setting I should be using or is there another way to generate a countdown from 30:00 to 0:00? Thanks for your help.

Duane