Are you burning to SL discs or DL discs? I ask because at 50% it would do a layer break on DL media.
My guess would a faulty drive actually so if it's in warranty I'd return it. Likely a power supply problem with the burner would be my hunch.
What would happen if you did a transfer rate test in Nero's CD-DVD Speed. This would push the rpm right up just as burning would do.