This is a wild guess here, but are you using a completely different version of hardware-accelerated display drivers now that you're running Slackware?
I ask this as it seems to occur only when you're streaming media online--and that's an activity that might very well include accelerated video drivers... or perhaps Adobe Flashplayer. Many streaming services utilize flash, so it's possible it's related.
I've heard some horror stories about Flashplayer on linux but, luckily, it's worked well for me overall, including on my Android phone.
Presuming you're running Firefox (not sure how things work w/Chrome), you could try flushing your flash cache/cookies and see if that helps: http://www.simplehelp.net/2010/09/16...r-flash-cache/
Hope this helps.