Transmit midi clock to guitar effect for tempo sync, seeing varying tempo?

I'm not sure if I am hearing things but.  When I setup Sonar to transmit midi clock to a guitar effect (Boss gt001) the time based effect sounds like the tempo is being changed.  I think I've seen this before with the Pod HD500 actually showing the tempo change.  I think its a very small change but the effect on the sound is very unpleasing.  This may be due to the guitar effect not handling slight tempo changes nicely.  Has anyone else seen tempo variations?  Note the project is not changing tempo and by slight I am thinking (2 to .1 bpm).
 
Thanks,
Ken