Here are some things to check:
FCP X 10.0.3 runs very well on my 2009 MacPro 2 x 2.26 GHz Quad-Core Intel Xeon with 16 GB RAM and ATI Radeon HD 5870 1024 MB
First, check the spec of your iMac against the system requirements:
Particularly your graphics card. If it's listed here, it's not suitable:
If you are getting crashes, there is some conflict on the OS. Create a new (admin) user account on your system and use FCP X from there - if it runs a lot better, there's a conflict and a clean install would be recommended.
Other ways to improve performance:
Keep projects to 20 mins or less. If you have a long project, work on 20 min sections then paste these into a final project for export.
Create Optimised media - most camera native files are highly compressed and need a great deal of processor power to play back - particularly if you add titles, filters or effects. ProRes 422 takes up much more hard drive space but is very lightly compressed. It edits and plays back superbly.
Hide Waveforms at all times when you don't need them (both in Browser and Storyline / Timeline). They take up a lot of processor power.
Create folders in the Project and Events libraries and put any projects you are not working on currently, in those folders. This will help a lot.
Move your Projects and Events to an external HD (make sure it's formatted OS Extended - with journaled on or off) and run from there.
The biggest single improvement I saw in performance was when I upgraded the RAM from 8 GB to 16.