Guitar cuts out suddenly in Garageband

I'm playing my irig HD 2 through Garageband on iPad and it sounds great. However when I play a chord or note and let it sustain (usually in a metal type of tone), it suddenly cuts out as the sustain fades. I've played with the gain control on the irig, the input level and noise gate in garageband and the volume on the guitar in practically every combination to no avail. One other point to note is that this doesn't happen when playing through amplitube.

Any thoughts on how this can be corrected?