Matt,
Which video camcorder do you have? Is it HD?
If you can provide a short file (30 seconds or so to keep transfer times down) on some site where I can download the original, I'll see if I can import it, play it, put it on the timeline and trim it, and generally see if PD7 on my system will handle it.
Also it will help if you list your editing computer specs such as processor and clock speed, amount of RAM, and graphics card.
Each one of these can have some bearing on problems such as you describe. It is extremely unlikely the trial version has any issues that bear on your problem.
I just imported a 1 min 20 sec sample MP4 QT file shot with a Panasonic Lumix GH1, dropped it on the timeline, split it and removed a segment and it plays just fine. I'm using a new Core i7 processor based computer with an ATI 4670 512MB graphics card