If they are random, most probably mania from Bipolar disorder. However, when I have flashbacks, my thoughts race as a panic attack sets in.

I have a journal and write whenever this happens because my thoughts slow down as I'm trying to get it on paper or it's interesting to look at afterwards to distinguish what just happened in my mind.

But it may be different for others.

Yes, racing thoughts are common in PTSD. They are a part of your anxiety. When my racing thoughts were bad when I was in the hospital, I was medicated with antipsychotics. Benzos work as well. You can also do things to regulate the speed of your thoughts such as listening to a song with a constant beat over and over (yes I know it gets repetitive, boring, and maybe even annoying, but it works for me) Good Luck, as I know the racing thoughts can be a bear to deal with.

Writing down your thoughts, as mentioned, is great. I sometimes clean my house, exercise, or listen to music. At one point I was reading a book a day to occupy my mind. Hang in there. :)