This seems to be a problem on some computers with Intel graphics cards and Lubuntu. Only follow this if you're using an Intel graphics card.
I had the same exact problem with my kids laptop running Lubuntu 13.04. I installed, and re-installed flash, tried an older version of flash, and other things, and nothing seems to work. All I had to do was to download and install the latest graphics driver. In my case it wan an Intel built in graphics adapter.
Note: I have Lubuntu install updates, and other restricted during installation, and I didn't have to do anything with installing flash.
Download this driver from Intel. Go for the latest version but make sure that you get the right architecture (32 vs 64 bit). You can check this in a terminal by running
uname -i. If it says x86_64, you want the 64 bit version, otherwise assume you need the 32 bit version.
Then to install it, just press Control+Alt+T on your keyboard to open Terminal. When it opens, navigate to the Downloads folder, and run the command(s) below:
sudo dpkg -i <file_name>.deb
Once installed, go under the menu, and click on the Intel driver installer. I believe it under the accessories menu, but I'm not sure, just look for it. Once it download the needed files, reboot the system, and see.