Have you tried swapping things around one at a time and observing the results to see if the problem follows a particular piece of gear in the chain? It may help you narrow down the problem. For example:
[01] switch L/R caps (if possible)
[02] switch L/R mic cables
[03] replace mic cables outright
[04] switch L/R inputs into pre
[05] switch L/R cables into ADC
[06] replace cables into ADC outright
[07] replace cable out of ADC to recording device
[08] replace cable from playback deck to soundcard
[09] replace recording software
[10] replace the recording device (maybe use your PC directly instead of DAT)
[11] transfer through a different soundcard
Edit: Oh! One more thing - if it were me, I would be worried about it b/c it may be indicative of an impending device failure. Maybe not...but I'd still wanna nail down at least what piece of gear is causing the problem and then contact the manufacturer about it (if it's something not easily replaceable, like mics, pre, ADC, etc. as opposed to cables, soundcard, etc.).
Edit 2: Must...wake up...before posting. As an example, my modSBM-1 was truncating WAVs at about -6dB, ruler flat, for some reason. I didn't find this out until I viewed the WAV in an editor b/c it basically sounded fine, but turns out my op amps were fried for some reason and would've continued to get worse. Fortunately, the Oade Bros replaced them for me free of charge.