Why does Mac App Store log me out in the middle of a download of Lion and "resume" won't work?

Question

I have previously purchased a copy of OS Lion and have been loading it onto each of three Macs in our home network.  This worked fine for an iMac G7 and iMac G5, but is failing for a White Macbook (running Snow Leopard 10.6.8).  Other apps have been successfully downloaded on the Macbook, so the problem seems to be only associated with Lion (which is a 3-4hr dowload).
When I log onto my Mac App Store account and select "Purchases", my prior purchase of Lion is there as it should be.  I then select "Install" and it starts to download as expected.  Then about 10 minutes into the download, it pops-up that it is going to log out and offers me the option to click "cancel" (within so many seconds) to elect not to log out.  If I select "cancel", it continues to download normally until about an hour later when the same thing occurs, and so on.  If I do not select "cencel", then it logs out and when I log back in "Resume" doesn't work in App Store.  It shows that zero GB have been downloaded when at least 1GB have already been downloaded.
So, unless I sit there vigilantly watching the download for 4 hours to select "cancel" whenever it pops up. I just can't download the software this way.  With a capped data plan, this has the potential to be very expensive.
Btw, I have tried the simple thing of switching from a WIFI connection to cable, but this had no impact on the problem.
Can anyone help?

Answer 1

but is failing for a White Macbook (running Snow Leopard 10.6.8)
Launch iTunes. From the menu bar click Store / Authorize This Computer.
Try again.