MIDI Notes Not Being Triggered After Being Looped?

I thought this problem was originally an issue with the Step Sequencer as this is where I first came across it but after more investigation I am getting exactly the same issue going directly to Session Drummer 3 and Dimension Pro
 
 
If I set a 1 bar MIDI drum pattern to play as a loop it will play fine the first few times
 
However, after a number of loops (usually after 6 or more) one or more of the drum notes are not triggered
 
In my test example it was the last beat of the bar missing - the beat does not sound and the drum does not "move" in Session Drummer - so I can only assume that this MIDI note is not being triggered
 
 
Any ideas as to what could be happening here!!?!?!?!?!
 
why would it work perfectly the first few times and then just ignore some notes (usually the first or last beat in the bar during my tests) after are few loops?
 
I am running SONAR PE 8.5.3.282 on VISTA and using an M-Audio Fast Track Ultra
 
Any ideas?
 
Fanks
 
Paul