NULL queries getting stuck and using up connections

Running show processlist in phpmyadmin for me often shows queries with an Info of NULL and a Time of apparently a few seconds, but which have in fact been stuck there for a long time (each time I run show processlist the same Id may have a different Time). I have no idea what these queries are or why they get stuck, but they can often cause all my available connections to be used up and stuck on waiting in apache status, causing sites not to load. Is there a setting I can change to terminate queries that are stuck for a long time? Is the php.ini setting max_execution_time responsible for this (I had it set to 3000 from the default 90). I have attached a screenshot of running show processlist.