Intermitent sound drop out new X1c project

Just been working on my first 'serious' project in X1c. I have superior Drummer 2, EZ Drummer Percusion, accoustic guitar, two electric and a base line recorded. For FX I have two instances of POD Farm 2 64Bit on the electrics, and on Bus I have Vintage Channel x 1, cakewalks  Boost  on 2 busses all for drums and Sonitas reverb on another Bus.
The issue is when mixing, every so often the drums drop a beat, not in timing but just no audio, seems to be the BD. This is if I check the midi events the beat is there, with sufficient velocity and volume. I had also created an audio track for the BD,before this happened to help line up timing of the base and guitars, find it easier to work with the wave form than the midi events. The same beat drops out on this track too, so does not appear to be a midi or Superior issue.
Has any one else come across this.