The purpose of this study is to compare the effects, good and/or bad, of not adding bevacizumab or of adding either a lower dose or a higher dose of bevacizumab to cetuximab plus an irinotecan-based treatment for colorectal cancer. The standard treatment is chemotherapy with a combination of drugs that includes irinotecan and cetuximab.  The research part of this study is the continuation of bevacizumab in lower doses, higher doses or not at all. We want to know the effects of these doses on patients and their colorectal cancer to find out which is better.