What are your system stats? CPU, RAM, Video Card, etc... The reason I am asking is that it sounds like a video issue. Inventor can be pretty picky about your system components. Autodesk should have a list of compatable hardware on their website, although if you are using Iv6 they may not. All in all it doesn't sound like it is anything you are doing wrong if that is any consolation. Is this your personal copy of inventor or a corporate copy? If it is yours, you may want to reinstall it. If it is your company's, you may want IT to come take a look. BUT DON'T DO EITHER YET! We may still be able to fix you without that.

And just so you know...no...this is not a normal issue with inventor that I know of. I have used inventor for several years now without ever having a similar problem.