roller coaster 
noun
1A fairground attraction that consists of a light railway track which has many tight turns and steep slopes on which people ride in small, fast open carriages: [as modifier]: a roller-coaster ride...
- Jimmy and I liked fast rides like the roller coaster.
- It felt like when you're riding a really fast roller coaster, only a hundred times worse.
- But we rode a roller coaster in Toronto with her on the fairgrounds after the show.
1.1Something characterized by wild and unpredictable changes: a terrific roller coaster of a book...
- It then started its wild roller-coaster ride, first up well into double digits by 1981, then down to zero, a move that twenty years later is still in progress.
- Having a teenager in the family can mean you're in for an emotional roller-coaster ride.
- The trademark roller-coaster narrative has been replaced by something more subtler, more powerful, but lacking none of the ambition or scope.
verb
(roller-coaster , also roller-coast) [no object] Move, change, or occur in a dramatically changeable manner: the twentieth century fades behind us and history roller-coasters on...
- The raft roller-coastered, bounced through the rapids, slammed into turbulent water.
- And then he is off, roller-coasting through all the live issues in education, a man who is not afraid to say what he thinks and who brings huge energy and passion to his opinions.
- Try roller-coasting over Turnhouse Hill, Carnethy Hill and Scald Law, and let the wide open skies and the distant horizons exhilarate you.