Not able to access HTTPS websites

I am having problems accessing certain(not all) SSL websites behind a FortiGate 110c running 4.0 MR2. Sites I cannot access include gmail.com and a local banking website. It is quite odd because last week I was unable to access gmail.com and Firefox would give the error: " Connection Interrupted" . Today in the morning I was able to access the sites for a few hours, however, within an hour from the last successful access I could no longer access the sites and Firefox would give the error: " The connection was reset." During this time-frame no changes were made to the FortiGate - which is weird why all of the sudden the sites would go from not working, to working, then back to not working. Anyone have any idea what could be causing this or where to start troubleshooting?