I tried the 4.3.1 upgrade and it caused more problems than it solved. In 4.3.1 if desktop effects are turned off the menu will stay open on shut down and restart. Also, with 4.3.1 I lost the ability to log off, the system would just hang. Not all of the needed packages are supplied for the upgrade to resolve some other issues too, I read that on the Kubuntu forum.
With KDE 4.2.4 I have a nearly perfect working system, with the exception of Kate (Works but likes to crash on exit...) I also had a problem with the screen saver not starting reliably, the solution was to set it's priority to medium, now it starts every time. The issue with Kate was resolved in 4.3.1 but the fix was never ported to 4.2.4... I just use another text editor to just avoid the problem.
You might be able to get flash to use hardware acceleration to resolve your problem. I used this method when I tested KDE 4.3.1 but I have Intel graphics. Currently I am running KDE 4.2.4 with the 2.26.30 RC3 kernel with the Intel 2.28 driver and running the fixmtrr script at start up to resolve the Intel issue, so I no longer need to use hardware acceleration for flash.
I will stick with KDE 4.2.4 until more bugs are fixed upstream, KDE 4.4 development has already begun.