When I click on a video in windows explorerinside vegas with preview autoplay enabled, all my video plays fine and at full frame rate whatever I throw at it. if i drag it to the timeline it will not play full framerate.. Does the explorer window utilize the GPU?
Which version of Vegas are you running?
What basic system specs?
I don't see why the timeline is diff than auto-preview. Unless it is something to do with project properties being way off from the media you are using. Check your media vs the project properties (ALT+enter)
I'm not knowledgable on this but I seem to remember that Vegas does not use your GPU for rendering or preview. That's a shame if you have some beast of a Video card. I think it's all CPU dependent.
It sounds like some other processing is taking up your cycles. Do you have any plugins/effects running over your tracks or output? My system will often lose frames if for example I use the color corrector over a track.