More details. What version and OS? What exactly do you mean by zooms back out?
Check your playback control palette and your comp settings. you may have set some option to always play at full res or in fullscreen and going back to the normal view resets the TL zoom...
Mylenium: I checked the preview section and the comp settings but there isn't anything different to how I normally work and I've never had this problem before.
Rick: It's the timeline that zooms out, to see the whole 7:30 worth of layers. Normally I'm zoomed in to edit each layer but after a RAM preview, the timeline zooms back out to view the whole length of the comp. And I'm running OS X 10.8.4 with CS6.