Live View uses a completely different focusing system from the normal 19 point array of the camera. LV uses the image sensor itself and a contrast detection method that's much slower than the normal focusing method. The normal or standard focusing method using the 19 points seen through the viewfinder is phase detection method.
Since you were using a 400/5.6, IS isn't the problem, since the lens doesn't have stabilization. However with a long lens like that (especially one without IS and on a crop sensor camera), the problem is usually camera shake. The 7D and other 18MP cameras are more sensitive to camera shake, possibly due to the very densely packed pixel sites on the sensor. Canon put out a white paper on this a while back and recommended trying to use sllightly higher shutter speeds to offset it. Even using a tripod, camera shake can occur (due to a breeze or the user pressing the shutter release or even the slap of the mirror and movement of the shutter).
I don't use LV a lot with my 7Ds (both of which are from the first batch, were pre-ordered and deliverred the first few weeks the camera was available). I had a bad keeper rate the first few months using the cameras, but it was user error. Once I stopped trying to use all the fancy AF modes in ways they weren't intended, simplified things, I get a keeper rate that's very high now. I probably cull out 5% or less images for focus issues. My keeper rate with my 7Ds -for focus alone - is better than I've ever gotten with any other Canon model (mostly used 5DII, 50D, 30D, 10D, EOS-3, Elan 7E, A2E... but some other models along the way too, intermittently: 1V, 1DII or III, 1Ds, a couple of the Rebel series).
If shooting RAW in particular, 7D images are "soft", likely due to a strong anti-alias filter. Some additional sharpening takes care of that. I use 60% or more stronger sharpening with 7D than I did with 50D, for example. Sometimes the image softness is mistaken for focus problems.
Are the lens or the camera new to you? Does the lens have a filter on it? Have you Micro Adjusted the lens (tho that shouldn't matter in LV)?
