You can't stop a movie render and restart it. The only way to stop and start renders is if you export to an image sequence, this will allow you to pickup where you left off, or split up render jobs. You can then easily use the sequence to generate a movie in Motion or Compressor, it will go pretty fast. It's an old trick from the 3D rendering world.
If you're rendering for a playback in Motion with RAM Preview, you're only going to get as much as your RAM allows and no more. You might get a few extra seconds if you Clear RAM Preview first, but don't count on it. The only way to see a complete render for more than a few seconds is to Export and play the exported file in another player (QT Player is fine.) By default, Motion is set up to export ProRes 4444, so for the "preview" exports, switch that to Proxy or LT (it will play better -- the bandwidth on the higher quality PRs can sometimes choke QTplayer [I use QT7, so that might have something to do with it].)
Since Motion 5, RAM Preview actually does less than it used to. My guess is because Motion is now 32-bit all the time. You can set shorter Play Ranges for the parts you need to review...say, about 5 seconds... if you can get used to doing that in your workflow.