Sounds like heat to me. iMac slottie - passively cooled and notorious for heat issues. Effect is at the top of the screen (thus, top of the case) and coincides with a system freeze. Only happens on a browser (CPU/RAM pig) and Facebook (CPU/RAM pig). Try installing a menubar temperature and CPU load monitor(s) and watch what happens.
How much RAM is installed?
How much RAM is installed?
RIGHT NOW, make sure the bottom of the iMac is propped up to allow cool air in at the base. Pull the foot under the front forward, and prop the rear up on a chunk of wood or something, without blocking the intakes. If that helps, then heat is your culprit. It may also be time to clean and replenish the CPU heatsink paste, and even install an extractor fan.