I Have the same problem, but i´m in Pal land, so drop frames is not the issue here.
I have 20 captured XDcam-discs, recorded in IMX50m but captured via Firewire in DV-codec (Don´t ask me why), 5 of the disk have dropped the timecode on every clip (now starting on 00.00.00.00)
There´s no timecode breaks on the discs, and though the audio is recordet in 48Khz, FCP is warning me about different audio samplerate.
Any ideas?
Setup: Imac 24″ 3.02, 4Gb ram – FCP 6.0.4 -2TB Stardom disk raid