I've just done this with mine and it does the same. I think it's due to some kind of "touch-counter", a counter which seems to top out after you've moved the same finger around on the screen for a long time without breaking contact.
If I do this with a single finger until it stops responding, but then use a second finger (whilst leaving the first one in contact with the screen), the second finger is responded to and the first is ignored.
If you then repeat the rapid motion with the second finger, it too seems to reach the limit of it's 'counter' and also stops responding.
You can then repeat with a third finger etc...
In light of this I'm thinking it's a software issue and not hardware.
By the way, I now have cramp in my hand