I had the same issue (but limited to youtube videos), regardless if I was playing flash videos with Safari, Chrome or Firefox...
here's how I managed to solve it:
first make sure to play all videos that are available as html5 videos too to be played without the flash plugin to limit this problem as much as possible:
click on 'Join the HTML5 Trial' at the end of this page:
next block all advertisements from youtube - this is the key solution!
open up Terminal from Applications/Utilities...
enter the following line then hit Enter
sudo pico /etc/hosts
type in your password (the cursor will not move) and hit Enter
in the appearing screen scroll down to the bottom and copy+paste the host list from the end of my post
Make sure to leave an empty line at the very end.
Save your changes with CTRL+O
Exit the pico editor with CTRL+X
Relaunch your browser and test if the freezing simptome has gone.
this is the list (not all lines are required, but you can leave it as it is to minimise online ads)
#GOOGLEADS
127.0.0.1 pagead.googlesyndication.com
127.0.0.1 pagead2.googlesyndication.com
127.0.0.1 googleads.g.doubleclick.net
127.0.0.1 ad.doubleclick.net
#BIDVERTISER
127.0.0.1 bdv.bidvertiser.com
127.0.0.1 ads.bidvertiser.com
# ADBRITE
127.0.0.1 1.adbrite.com
127.0.0.1 2.adbrite.com
127.0.0.1 3.adbrite.com
127.0.0.1 4.adbrite.com
127.0.0.1 ads.adbrite.com
127.0.0.1 b1.adbrite.com
127.0.0.1 bstats.adbrite.com
127.0.0.1 click.adbrite.com
127.0.0.1 files.adbrite.com
127.0.0.1 pic.adbrite.com
127.0.0.1 vid.adbrite.com
127.0.0.1 site.www.adbrite.com
127.0.0.1 stats.adbrite.com
127.0.0.1 www.adbrite.com
127.0.0.1 adbrite.122.2o7.net
#CHITIKA
127.0.0.1 ads.chitika.net
127.0.0.1 ads1.chitika.net
127.0.0.1 blogads.chitika.net
127.0.0.1 ca.chitika.net
127.0.0.1 mm.chitika.net
127.0.0.1 parking.chitika.net
127.0.0.1 scripts.chitika.net
#ADTOLL
127.0.0.1 adserve.adtoll.com
#CLICKSOR
127.0.0.1 clicksor.com
127.0.0.1 ads.clicksor.com
127.0.0.1 main.clicksor.com
127.0.0.1 mci12.clicksor.com
127.0.0.1 search.clicksor.com
127.0.0.1 serw.clicksor.com
127.0.0.1 track.clicksor.com
127.0.0.1 www.clicksor.com
127.0.0.1 mp.clicksor.net
#YAHOO
127.0.0.1 adserver.yahoo.com
#TARGETPOINT
127.0.0.1 dev.targetpoint.com
127.0.0.1 srs.targetpoint.com
#FASTCLICK
127.0.0.1 fastclick.com
127.0.0.1 www.fastclick.com #[SunBelt.FastClick.com]
127.0.0.1 cdn.fastclick.net
127.0.0.1 code.fastclick.net
127.0.0.1 images.fastclick.net #[Panda.Spyware:Cookie/FastClick]
127.0.0.1 media.fastclick.net
127.0.0.1 secure.fastclick.net #[Tenebril.Tracking.Cookie]
127.0.0.1 sp.fastclick.net
127.0.0.1 fastclick.com.edgesuite.net #[a1795.g.akamai.net]
#BLOGADS
127.0.0.1 b.blogads.com
127.0.0.1 banners.blogads.com
127.0.0.1 c.blogads.com
127.0.0.1 cache.blogads.com
127.0.0.1 d.blogads.com
127.0.0.1 f.blogads.com
127.0.0.1 img.blogads.com
127.0.0.1 st.blogads.com #[WebBug]
127.0.0.1 stat.blogads.com #[WebBug]
127.0.0.1 weblog.blogads.com
127.0.0.1 z.blogads.com
127.0.0.1 www.blogads.com
#ATDMT
127.0.0.1 c.atdmt.com
127.0.0.1 click.atdmt.com #[McAfee.Cookie-Atdmt][Google.Diagnostic]
# 127.0.0.1 clk.atdmt.com #[disabled affects MS downloads]
127.0.0.1 ec.atdmt.com #[atlasdmt.vo.msecnd.net]
127.0.0.1 image.atdmt.com
127.0.0.1 ll.atdmt.com #[atlasdmt.vo.llnwd.net]
127.0.0.1 mir.atdmt.com #[p.mii.instacontent.net]
127.0.0.1 nc.atdmt.com #[EventTracking]
127.0.0.1 rmd.atdmt.com #[a898.x.akamai.net]
127.0.0.1 spd.atdmt.com #[a796.x.akamai.net]
127.0.0.1 spe.atdmt.com
127.0.0.1 srch.atdmt.com
# 127.0.0.1 switch.atdmt.com #[disabled affects Hotmail signup]
127.0.0.1 view.atdmt.com #[eTrust.Tracking.Cookie]
127.0.0.1 www.atdmt.com
127.0.0.1 partypoker.com
127.0.0.1 www.partypoker.com
127.0.0.1 www.casino.com
