Thanks for the tips, Stefan.
I took a closer look and it seems that a frame rate setting caused the issue:
10/3/2013 8:44:49 AM, Version: 10.0.0.59
10/3/2013 8:44:49 AM, Size: 1920x1080
10/3/2013 8:44:49 AM, System: NTSC 59.94i or 29.97p
10/3/2013 8:44:49 AM, Codec: Decklink MJPEG Compressor
10/3/2013 8:44:49 AM, AudioDevice: vMix Audio
10/3/2013 8:44:49 AM, AudioFormat: Default
10/3/2013 8:44:49 AM, AudioDelay: -84
10/3/2013 8:44:49 AM, RestartInterval: 0
10/3/2013 8:44:49 AM, StartRecording
10/3/2013 10:43:55 AM, StopRecording
10/3/2013 10:43:55 AM, Duration: 01:59:07
10/3/2013 10:43:55 AM, Frames: 205082
10/3/2013 10:43:55 AM, DroppedFrames: 9105
10/3/2013 10:43:55 AM, DueToSourceFrameRate: 9105
10/3/2013 10:43:55 AM, DueToHardDiskOrCPU: 0
10/3/2013 10:43:55 AM, DroppedSamples: 1
10/3/2013 10:43:55 AM, MissedSamples: 0
10/3/2013 10:43:55 AM, SyncedSamples: 0
10/3/2013 10:43:55 AM, AudioOffset: 0
10/3/2013 10:43:55 AM, AudioDelay: -84
10/3/2013 10:43:55 AM, AudioEngine
10/3/2013 10:43:55 AM, Running: True
10/3/2013 10:43:55 AM, SampleRate: 48000hz 16 Bit
10/3/2013 10:43:55 AM, MixingErrors: 0
10/3/2013 10:43:55 AM, Streams
10/3/2013 10:43:55 AM, Stream0
10/3/2013 10:43:55 AM, Samples: 0
10/3/2013 10:43:55 AM, Dropped: 0
10/3/2013 10:43:55 AM, Latency: 0
10/3/2013 10:43:55 AM, Stream1
10/3/2013 10:43:55 AM, Samples: 388055
10/3/2013 10:43:55 AM, Dropped: 0
10/3/2013 10:43:55 AM, Latency: 2
10/3/2013 10:43:55 AM, AudioInputs
However, I still have reservations about my current GPU - I'll do some stress tests and monitor it's usage stats.