Video display disappears until restart

Hello. I have been experiencing a sporadic problem with video playback for several months now, and I could really use some advice.

If I watch enough (locally saved) videos consecutively without restarting my computer, at some seemingly random point, the media player will stop displaying the visual component when I open new ones. Once this happens, the problem will persist until I restart the computer. Each time I try to open a new video file, the player will briefly expand to the proper display size as if the video portion is going to come up as it should, and then the player will just revert to the small size it would as if it were just playing an audio file. And the audio continues to work properly, but I won't be able to get the visual component to work until the computer is restarted. Interestingly, I can continue to watch videos embedded in websites without any problems, just not anything saved to my hard drive(s).

Generally, I use VLC Media Player to play music and watch videos. But once the videos stop displaying properly, I get the same results regardless of which media player I try to open videos with.

Other than that, my desktop runs perfectly. I generally have lots of applications opened up simultaneously, so I leave it on pretty much all the time (except when I have to restart it to fix this video problem). Although I haven't noticed any real pattern to when videos files stop opening properly, I would say I can usually open an average of about 20 videos before the problem occurs. It seems like I'm able to extend that number if I pause videos before they finish or if I open new ones while another one is still playing. But if I play audio files or let the videos play out to the end (when the video display minimizes), it seems more likely that the next video I open will cause the problem to reoccur.

Sorry it's kind of difficult to explain this issue (and I've had a heck of a time trying to search for solutions), so please let me know if there is anything I can say or do to help narrow down the problem. Thanks.

Basic Specs:
64-bit Windows 7 Ultimate (Build 7601 - Service Pack 1)
AMD Phenom 9950 2.6GHz Quad-Core w/ 8GB of RAM
NVIDIA Quadro FX 4500 video card w/ latest drivers