You can also try the following:
Switch off at the wall first - but leave the plug in to maintain the earth connection.
Check that the case vents are clear and not clogged with dust.
Open up the case and inspect for dust. If there is any blow this away with a can of compressed air. Do not be tempted to use a vacuum cleaner as, although you still have an earth connection to your M/Board, the vacuum can produce large amounts of static which can damage sensitive ICs.
With the side off ( be careful here ) switch back on and boot up.
Are all of the fans spinning fast?
if you leave the side off - does the "shutdown" problem still occur?