Hi Peter!
1) Be sure that your composition is based on the requested framerate used by your clock.
By Default, a manual clock used the requested framerate of the preferences:
so your composition may have the same framerate (wich might not be your case).
2) For my point of view, it is better to use the 
as 
link from instead of the "seonds" direct parameter.
Here is a composition that work well.
countdown.compo (109.9 KB)
3) You will encounter perfomances issues by linking a text to a clock. Everytime the TC change, Smode will have to recalculate the vector of the font you use.
Best way to do a clock: create a 
that contain every characters in vertical like that:
00
01
02
...
59
Then apply a, 
placement mask in order to display only a single second value and a 
2D transform . link the Y position to the decompose minutes parameter to make it move for the seconds.
i hope it will help you!
I forward the problem you encounter to the dev team to see what we can improve.
Thanks for report 
