Well, if lag manifest only as a car going back and forward then it wasn't lag. The viewer gets slower as more laps pass, usually after 2/3 of the race, but it happens always. When it happens i just close the viewer and open it again, that fixes it.
That's good news in terms of the update, not so good for your experience. But I can confirm that's a Java/viewer issue. We want to rebuild the viewers on another platform altogether (HTML5 most likely) as soon as we can, but our hands are tied with other work at the moment. If there are any HTML5 canvas developers reading this - get in touch! We would love any help we can get.
Which version of Java are you using? Also, is there a lot of chat in your races? I've heard in the past that lots of ongoing chat can potentially cause the viewer to freeze after a few minutes, although it's rare.
I use java 1.7 as suggested, but it also happens with 1.8. And yes, we have a lot of chat in races, and usually all of us experience this issue (sometimes someone havent it, but almost everyone restart the viewer at 2/3 of the race)