Hi. I think I may have your solution! Safari was driving me nuts on my brand new MacBook Pro and I was having the same problem of messed up/jumbled text on certain web sites. I couldn't even place an order on certain sites like Amazon or make a hotel reservation, for example, because the fill-in boxes had text all over them. I finally called technical support and escalated my call to one of the senior advisers. At last, she came up with a solution. The issue (at least for me) was in Safari preferences. Some of the preferences that I had changed over time were somehow interfering with how certain web sites appeared.
In summary, you have to delete any .plist files which have the word "Safari" in them. For me, these files were:
com.apple.Safari.plist
com.apple.Safari.RSS.plist
Once I deleted these and rebooted my MacBook Pro, Safari was returned to it's original state as it appeared when I first got the computer (i.e. the "Welcome" screen, etc.). This completedly solved my problem, and now all of my web sites are looking great with no jumbled text.
To find these .plist files, do the following:
1) Open the Finder window
2) Under "Places", click on the house icon (i.e. your hard drive icon)
3) Go to Library ----> Preferences and then expand the list of files that pops up so that you can read them all. Look for anything that has the word Safari in it (i.e. the two files that I mention above). Drag them to the Trash.
4) Close all windows and restart your computer.
5) After the Restart, open Safari. You will be back at the default Safari "Welcome" screen that first appeared when you got your computer. Try navigating to the troublesome web sites. I hope you will find that your problem is solved. It worked for me!