Hi,
The counters on the board are highly sensitive to noise. The slightest transients can trigger the counter. What you can do in order to isolate the issue - whether it is hardware or software, - is to try some of the shipping examples in LabVIEW. If the counter shows the same behavior, then definitely this is a hardware issue. Also, you can try the card on another system, and see if you get the same result. Lastly, make sure you are not close to noise sources.
Regards,
RamziH.