When some of your apps start playing up, try cleaning caches.

The Shift key might get you out of troubles, quickly. Going back to history, seasoned users who used System 7 might even remember that they could bypass loading extensions during start.

... it doesn't always need to be a hardware fault, or dust blocking vents. It could just be a software issue which makes your CPU running warm, so the system needs to cool it down. Go to Applications > Utilities ( a shortcut from Finder is cmd-shift-U) and find Activity Viewer. Then set the menu View > All Processes. Then keep watching CPU pane with processes sorted from the highest %CPU to the lowest. The “rogue” processes would appear on the top. Typically, the CPU load of Office programs and usual stuff such as mail is quite low. Safari can be diverse - some...