If you feed sound to the Epic, then take the R3D file, debayer it through RedcineXPro it to the same format as the Pix, then load up both files into your editing programs, are the files identical?
In other words, can you determine where the error is happening? Is the Pix wrong and the Epic is right, or is the Pix right and the Epic is wrong?
Me personally, if it were 1 frame, I wouldn't care in post and I'd just tell the assistant editor to fix it. The key for me would be, the Pix file has to be 100% identical to the Epic file. Without that, all bets are off.
If the Epic file is 1 frame out of sync with the Pix file, then you could potentially have a massive problem with the conform. What I think is happening is, you have a non-referenced timecode phase issue where at certain times, the timecode number slips over to the next frame. I have seen this happened with two non-genlocked cameras, particularly in 3D situations.
Also, are you basing sync on numbers on a timecode slate? Shutter speed (angle) can affect how the pickup "sees" the timecode number, so certain numbers may be interpreted differently on two cameras, side by side. This was -- and is -- frequently a problem in film post, and is one reason why we use the clap as "real" sync, and only use slate numbers to get us close. The clap is real-world sync, held as an absolute.
I'd also be curious to see what would happen in an all-23.98 situation. I've only done shoots and post sessions with 23.98 material, so there's always the possibility that something is a little off in a 25.00 world.