Grasping at straws, but.....You say there are no plugins except Superior Drummer....Are there any sends in use? If so, make sure that each send actually goes somewhere. Every time I have had oddball latency, it turned out that I had a send on a track(any track in the session) that had no destination. This caused a bit over 1900 samples of bogus ADC. Fast way to check; make all sends inactive and test
