Any other places you use TimerEventProcessor or Counter?
Anyway, you can not rely on the Event being exactly delivered one per second. The time may vary, and the system will not make sure the average time is correct.
So instead of _Counter, you should use:
// when starting the timer:
DateTime _started = DateTime.UtcNow;
// in TimerEventProcessor:
seconds = (DateTime.UtcNow-started).TotalSeconds;
Label.Text = seconds.ToString();
Note: this does not solve the Problem of TimerEventProcessor being called to often, or _Counter incremented to often. it merely masks it, but it is also the right way to do it.