I, too have marked sinus arrhythmia as does my 5 year old daughter. I've been told it's not bad, in fact it is a sign of a healthy heart because your heart is responding to variability in your breathing. Marked sinus arrhythmia just means your heart slows down a lot when you breathe out and speeds up a lot when you breathe in. HOpe this helps!
The medhelp heart forum doctor answered this specific question on 8/3/2006. Hope you can locate it. It is under the title "marked sinus arrhythmia"
:-)