Re: All versions of ubuntu
Maybe there is something wrong with your CD drive writing or reading. Although, if you tried writing them on different computers you would think something would work. If you boot a CD and hit any key when you first see symbols at the bottom of a purple screen, they should be a selection in the resulting text menu to check the (CD) disk itself against the checksum on the CD. That should usually find a corrupted disk (or read problems with the drive). Although, I have a CD/DVD drive on one old laptop that gets flaky after playing a DVD for about 90 minutes.
What video card or chip is on those computers. At least with nvidia, various Ubuntu versions sometimes require nomodset boot parameter only during install, sometimes only after install, or sometimes both. But it does not hurt to try that parameter in any case. The nomodeset parameter forces the use of user space video modules (which can be loaded/unloaded as needed) instead of the newer kernel mode setting.
If you changed memory in any of the computers you may want to download a separate iso for memtest86+ and run that for awhile to see if you have any memory issues. I have run across a couple of cases where memory that should have been compatible with a PC would have occasional issues and produced errors with memtest86+. Yet that same RAM lives on in another computer with no memtest errors.
i5 650 3.2 GHz, 8 GB, nvidia GTX 550 Ti 32" 1080p | i7-4700, 8 GB, Intel HD 4600/nvidia GTX 765M 15.6" 1080p | etc.