Database Optimization

My site is going down at peak hours doe to the database not being configured correctly. In phpmyadmin advisor there are some suggestion that i need them to be fixed 1. The query cache is considerably fragmented. [login to view URL]: Suboptimal caching method. Recommendation: You are using the MySQL Query cache with a fairly high traffic database. It might be worth considering to use memcached instead of the MySQL Query cache, especially if you have multiple slaves. Justification: The query cache is enabled and the server receives 1192 queries per second. This rule fires if there is more than 100 queries per second. [login to view URL]: The rate of reading the next table row is high. Recommendation: This indicates that many queries are doing full table scans. Add indexes where applicable. Justification: Rate of reading next table row: 914752.7 per second, this value should be less than 1 per hour [login to view URL]: There are too many joins without indexes. Recommendation: This means that joins are doing full table scans. Adding indexes for the columns being used in the join conditions will greatly speed up table joins Justification: Table joins average: 49.3 per second, this value should be less than 1 per hour [login to view URL] temporary tables are being written to disk instead of being kept in memory. There are some other suggestions in advisor. i Need the problem with database going too slow at peak hours to be fixed in general, if you have any suggestions on what is going wrong.