I have the same issue, iPad2 latest iOS, +5Gb free, airplane mode after reset, latency set to 512 in AB, no other apps running.
After recording 4-5 tracks (audio only, no midi, no virtual instruments, no effects), I get stuttering and crackling noices while recording, and tracks slipping off time at playback.
I’t too bad, because I really like working in Cubasis, and would prefer it but I’m stuck using GB for the moment. No problem recording 10-12 tracks or more, including virtual instruments and effects there.