If you are monitoring your Fast Servers you may have the QRProxy shown in a critical state while search query are working fine (ie returning results) and/or no events are logged on the Fast event log neiher on the SCOM console.

You may have an error event on the Fast event log which is saying “An unhandled exception occurred: Unable to connect to SAM” from time to time, or even only once.

In order to solve this issue, you can try the steps provided on this TechNet article http://technet.microsoft.com/en-us/library/ff393757.aspx