在Heroku上使用Symfony2进行随机高响应时间

I am receiving high response time errors within my Symfony2 application on Heroku in random time intervals. I activated New Relic to see what is happening in detail and it seems that the SessionHandlerProxy is doing something that takes very long. Does anybody have an idea where the problem comes from? Btw. I am using Redis for session handling but the Redis connection is stable without any down or slow response times.