Hi there,

That's an interesting observation. It's possible that you are just more excited to play during the first few minutes and your mind is fresh. I wouldn't worry about it too much. Learning and practicing is a physical thing and it's totally possible that your fingers/muscles/concentration is being worn down the more you practice something.

I have had this happen before and what usually works is to take a quick break and then jump back in. After the small break, which can be 5 or 10 minutes, bathroom break, etc, when you start practice again you will be sharp once more.

That's what I suggest, take a break!

-Billy