I realize my question has been asked before, usually phrased as "sometimes Ubuntu doesn't boot", but after reading some of the other questions and answers, I think my situation is a bit more complicated.
The problem: About once every 5, maybe 10 times I turn on my computer and chose to boot Ubuntu, it doesn't boot, but the screen turns "purple" or perhaps more precisely: it's empty/black, but backlight is on as far as I can tell.
In this state, I can do absolutely nothing. Ctrl Alt F1 to switch to a terminal doesn't work, so in the end, I have to force a power-down by pressing the power button.
My setup: I dual boot Ubuntu 13.04, 64 bit, and Windows 7 SP1, on a Lenovo Ideapad U310. The U310 has both an SSD and a HDD. Ubuntu's root directory and swap partition are on the SSD, my home folder and Windows are on the HDD.
Some additional info: the grub menu appears always, and I can chose whether I want to boot Windows or Ubuntu. The problem appears (sometimes) after I chose to boot Ubuntu -- the screen becomes black/purple, nothing else on it, and it stays like that indefinitely.
Any idea what could be the cause for that? If necessary, I can read/attach log files that might help discovering what the problem is, but I didn't know which log file to look into.
Any help would be appreciated.