Stop ( Not terminate ) EC2 Instance when CPU drops below certain level

I have two servers ( with my app on it ) already running.. created from Ubuntu AMI ! While using auto-scaling it starts new instances using an AMI.. Can I use auto-scaling to ONLY stop instance ( not terminate; so that I dont nee ami when starting server in future ) ; and later start instance ( old instance which I had stopped ) whenever CPU increases above x% !

If not auto-scaling; I am ok with any other solution also.. With Cloudwatch I can stop instance; but how will I start it back !