Thanks, QuickTimeKirk!
Well, a few minutes ago, I tried to run a file through StreamClip but got the same results. However, noting what was said on the Squared5 link, I think I may need to do something with DivX. I recently installed DivX 6 for Mac in hopes of resolving the issue. It looks as though I now need to install the 3ivx codec to fix THAT. Does that sound accurate? This is my first Mac. So, I'm sure one could guess that it is also my first foray into complicated codec problems. I'll try the 3ivx and/or XviD codecs and see what happens.