I'm running Slackware Current

jan@oxygen ~$ uname -a
Linux oxygen 3.18.11-smp #2 SMP Tue Apr 7 14:36:46 CDT 2015 i686 Intel(R) Core(TM) i3-4005U CPU @ 1.70GHz GenuineIntel GNU/Linux

on my new Lenovo G50-80 and all is fine. The only thing: sometimes, after long runs of seamonkey for web and mail, I cannot properly shutdown xfce4. The screen goes black and that's it. Alt-ctrl-backspace and the other hotkeys do not work. I need to Alt-Ctrl-Fn-sysreq-SUB to shutdown.

Anyone any clues?

Here's about installing Slackware to the Lenovo G50-80

That is an outdated version of Slackware-current. The reason I know this is the -current kernel is at 4.1.6. I'm not sure how far out of date you are, but the kernel was updated to 4.1.6 over a month ago and 3.18.11 was added back in April, so all we know is your install is from somewhere between April and August. -current is not considered a stable release and can have problems, but it is constantly updated. It is best to report problems on a fully updated installation, since the errors you mention may have already been fixed in those updates.

And since this doesn't cover the installation of Slackware, I'd probably create a new topic in the regular Slackware forum if the latest -current doesn't fix your problem. That forum is more active than this one, and more people would likely see your post.

I downloaded the latest version off ftp.belnet.be two weeks ago.

Well, they must've had an outdated ISO since the kernel was updated to 4.1.6 back on 21 AUG (almost 6 weeks ago). And any -current ISOs are unofficial and are not provided by Slackware. You should use slackpkg to update yourself to the latest version. If you're running Slackware-current, you should also be monitoring the changelog, so you can update when changes are presented.

If you want to get -current ISOs, Alien Bob's mirror might be a better option as he regularly updates it when -current is updated. But slackpkg is the recommended route to keep your -current up-to-date once it is actually installed.