I’m not sure it’s part of ADD. It’s certainly related to anxiety.
When your brain freezes you are doing well if you can remember your own name. The synapses are not firing. It is not a good thing.
I’ve had it twice, on my first test in college and on a big test in medical school. The strategy is to take a break and come back. My wife has it if you say the word “math”. The strategy is to call it something else and try not to use numbers.
If you have experience with brain freeze let me know please.