There seem to be a whole variety of reasons for crashes in turbo mode (you can search around or read the stickied thread on overclocking). A lot of people have had the same experience as you with stability at 900 but lockups at 1000.
There are a small number of reports of freezing resolved by the installation of cooling, but I really don’t know what to make of these. Experts say that there is no benefit, and I’m inclined to believe them. You can probably get some heatsinks for $4, so why not give it a try just for fun?