Richard's wife is having problems with the iPad intermittently freezing up as she's typing on the screen. It'll be unresponsive for a while and then catch up. But the Genius Bar can't find anything wrong with it. Leo says that intermittent problems are difficult to diagnose. Turn it all the way off. Then power it back on. That will reboot it.
Freezing up after a long period of time could be a clue to overheating. Or there's an app on it that's got a "memory leak" and won't release the RAM it uses after you stop using it. That's why shutting the iPad down and restarting it fixes the problem. So do a factory reset and don't restore it. Just use a clean boot. If you can't repeat the problem after that, then it's a software issue and you have to determine what app causes the problem. If you can repeat the problem, it's a hardware issue.