The grey exclamation point is the Out of Memory (OOM) icon. As we detect that you're running out of memory, we'll attempt to free as much stuff as possible, but if the content is allocating too much memory too fast, there's often nothing we can do to prevent the situation. Rather than crash unsafely, we halt execution and terminate the player.
If you're running a lot of simultaneous tabs, that's probably a contributing factor. If you have more than 2GB of RAM and you're running a 32-bit operating system, that will limit the amount of available RAM to 2GB. Sometimes content is just written inefficiently and it doesn't free resources that are no longer being used. We'll eventually force stuff to be deleted, but it's a last-ditch effort as garbage collection is CPU intensive and done at the wrong time, will cause games and video to hitch and stutter.
If you're getting the OOM icon on specific content, links would be useful. If it's happening all over the place, does a reboot at least make the problem go away for a while? If so, you might keep an eye on task manager to see if particular tasks are continuously growing over time. The Flash tasks will grow until they hit an upper limit, but then they should periodically drop (as garbage collection happens), then grow back to that upper threshold. If other processes are growing unbounded, or you're just hitting a piece of content and immediately getting the OOM icon, those are really interesting clues.