Mysql Replicate, Master updates lock Slaves selects

I have have a classifieds site with tons of traffic, have 1 master and 3 slaves. Every slave is a web server and uses his localhost mysql for select queries and master for writes. The app is developed in Rails 3.0.7 (Octopus for replicate) with MySQL 5.0.77 and I have a huge problem:

In moments of high traffic, the master replication (inserts, updates and deletes) locks completely the table that he is writing to. This is normal, I know, but query takes like 15 second开发者_JAVA技巧s some times locking every select, and it hurts me bad.