When you say you are losing a channel, do you mean you look down at the level display on your D8 and only see one level meter going? Or do you mean all looks fine when you are recording but one level is MIA when you listen on playback? Also, you mention that one channel reappeared when you transfered the show? Does this mean is wasn't there upon playback when you first tried and subsequently was there on the same deck when you played the tape to trasnfer?
I'm trying to get a sense of whether the problem is with your D8 or the tape, or if the problem is with the mics/jacks/cables. If the channel was MIA on one playback but there on the next, that would eliminate any problem with the mics/jacks/cables etc (i.e. it's obviously not a permanent flaw that took place when you recorded the show but rather this would lead me to believe you have a problem with your deck such as head alignment or worn heads or else a bunk batch of blanks).
The first thing I would suggest is to hook up your gear and crank your stereo up in your house and record some of it while you wiggle cables, jacks etc. all the way from the mics to the deck and see if you can make this problem occur. My guess is not based on your description but this is simple to do so you might as well check. Another thing I suggest you try is playing your master on some other decks - hopefully you have some friends who also have decks you can use - I suggest trying a few dfifferent makes/models if possible as I've had success transfering tapes in the past that gave me problems on my Sony portables that played fine on my R500 on Denon DTR-80P. Many of the pro decks will give you some sort of "error" message if your tape has issues (I know the R500 does this).
If you narrow it down to problems with your deck (more likely IMO then a batch of bad tapes), I suggest you try your best to get your tapes trasnfered before you send in your deck - I once sent in a deck for a similar problem and they either re-aligned or replaced the heads (I can't remember which) and many of the masters I had made with the problem heads would not play right on the properly functioning heads (even though most had played fine on the funky heads) - my best guess is that they were recorded out of alignment or something to that effect and would only play back properly on the same deck with the same heads.
If you do send it in, I personally wouldn't send it to anyone but ProDigital as they've always done me right for the past several years/several DAT decks.