Sounds similar to what I had to deal with yesterday. There are two avenues that you need to take -- what's causing the UDP flood and how it is being caused.
I suspect you need to look for created this week that container code similar to:
eval(urldecode(@$_REQUEST['encoded_bad_stuff']
We had 3 servers that had a few hundred of these files created through an exploit in Plesk Panel on Tuesday evening. Yesterday, those files were accessed and triggered a UDP flood on our network.
Talk to parallels support about the hotfix issued last night for 9.5.4 that supposedly resolves the sql injection vulnerability in Plesk Panel.