Plastic Window buffeting like crazy @ 50+ mph

[MConte05]

I bought a 2002 Boxster about a month ago from a dealer which had claimed they replaced the plastic window because it had cracked prior. I have driven it every single day with the top down (beautiful weather in St. Louis this year) except for today when it was balls hot, so I drove with the top up and the AC on.

Once I hit over 50mph on my way home from work, the plastic window just started buffeting like crazy, almost undriveable and was worried something might break... it was bad. Only way to relieve it was to put a little pressure on the top of the window/top

When I got home, I played around with the top in different positions, trying to see if there was somewhere that was causing play in the top. However it seems that the plastic window itself was the source of slop.

I have had the top up maybe twice before, because of light rain, and never experienced buffeting. Could this be because the plastic was getting soft due to the intense heat (well over 100 degrees) and sitting outside in my work parking lot? Or is something genuinely wrong with the plastic window?

Thanks!