Those errors don't say much, and unless someone is a really good DB Admin, then they aren't going to tell us anything at all because there's no idea of what is causing the issue, just that it's happening.
My first and probably biggest suggestion is that you should look at the slow query log in MySQL. That will show you what queries are taking the longest, and that shoudl give you an idea of what's slowing the server down.
I'd also suggest looking at a good caching system. There's plugins for it, but there's also some great server-side tools that can help a lot as well. That will definately help reduce the load on the DB.