Dilemma - Mixing In The Box

So, I'm in the middle of mixing an album for a AAA artist, and we're stuck on one song in particular. It's a full-band Rock track, and we're combining two different takes (done at different times with different players) together to make a "best of" take that really rocks. We have two sets of everything, including drums (20 tracks of drums!). Though we're not using everything all the time, there are sections that cut back and forth between the different drum takes. Individually, the drums sound pretty rockin', but when you start to add everything together the definition gets more and more hazy with each additional track, and pretty soon it sounds like I'm mixing on an old cassette 4 track. Worse, actually!

How do I get out of this one?

Here's a bit of back story - the tracks were all recorded professionally by professional engineers in reputable studios with quality gear. The players are all pro's and the songwriting is quite good. No excuses, right? Let's go a step further: this entire project was done in Pro Tools (24 bit, 48kHz WAV) and we're mixing on an HD3 rig (G5 dual 2 gig with 4 gigs of RAM) and not really pushing the limit of the DSP. This song is being mixed "in the box" (without any outboard mixer or processing) with a track count of 64, many of which are stereo tracks.

Here are the big questions:
1) Why does the mix sound like dookie when the individual (and even small group) tracks sound great?
2) Why does it get worse when I submix the drums or the vocals?
3) How do I fix the problem before the artist wigs out? Or before I yank my hair out?

And, oh by the way, the artist wants this done and mastered by the end of the week.

Clues? Hints? Suggestions? Send 'em on in! But tune in tomorrow and I'll tell you how I fixed the problem and beat the reaper, and how you can, too!

Au demain,

MBM