Often my pages (using PHP and MySQL) do not render completely. It get error "Fatal error: Maximum execution time of 30 seconds exceeded in...". Even cached Google pages by show this message. Other times the page loads in a fraction of a second. Also accessing database with phpMyAdmin gives this error.
The webhost said that SQL timeouts are almost always caused by blocking issues related to the implementation of the client or clients. One client could for example leave an update transaction open and thus block any other clients trying do perform a committed read.
I do not understand this. Can someone please give some more hints?
The webhost was not able to offer support for "obvious" reasons. Bad support! The site was hosted on a Windows Server. I changed to a Linux Server and my problem is solved now! It took me a few hours of work to move the site. My lesson: do not use MySQL/PHP on Windows server.