Economic Growth Theories - The Roller-Coaster of Growth - Why?

Economic growth tends to occur in a cyclical pattern. This pattern is called the trade cycle and it can be clearly seen when looking at the behaviour of growth over the last 20 years or so. There is a graph showing this in the explanation section on economic growth. So why do these cycles happen? There are various theories as to the causes:

• Random shocks - events often occur that are relatively unpredictable, but have a significant effect on the economy. Recent examples include the failure of banks in Japan, and the collapse in SE Asia. Both of these have affected the UK a great deal.
• Policy-induced - politicians have often been known to put in place policies to boost the economy. It would be cynical to suggest that this often occurs around election time, but it does. This can lead to booms which lead to the incoming government having to deflate to slow the economy down again.
• Imported cycles - if the rest of the world is growing in cycles, then this will affect on us. Our exports may fluctuate, which means that aggregate demand will change and therefore growth changes.
• Expectations - expectations can have a powerful effect on growth. For example if firms expect there to be a slowdown, they may delay investment plans. If they do that then aggregate demand will fall. If aggregate demand falls, so does growth. A self-fulfilling prophecy?!?
• Sunspot activity - there have also been suggestions that growth cycles are linked to cyclical sunspot activity, but it's quite possible that people suggesting this have been themselves affected by sunspot activity (or some illegal substance!).