Macbook Air 1.6; Something really weird!!

Hi! I have an Mac Air 1.6 mhz core 2 duo Intel CPU and 2 gb 667 ddr2, an x3100 video card with 144 DDR2 shared video memory. I tried to play some 1080p movies but some very weird happened. After I found a decent player, PLEX in this case, the videos played smoothly at start but after about 10 minutes of playing, the hard drive ( a PATA 4200RPM) started to spin and the image and sound began to deteriorate. This happened gradually and after a short time it was impossible to understand something from the movie...
I can't find any clue about this, maybe the HDD is too slow to bring all the data needed? Why at the start plays so good and after a short time it crushes? Or maybe is just overheating and the some sort of device lower the CPU speed?

...

what kind of surface is the comp on?
mine is a very recent model unibody and i keep it on an external fan (cooling pad) when watching video
heat will definitely cause video to "bleed" or have errors;
but if it consistently happens in a cooler environment, it is probably just the video itself, digital file or hardcopy
i had a copy of a movie that would play for a while then the picture would screw up, then if i just paused it in quicktime, it would start fine again;
probably just a low quality copy

Yes man you are right!! I can't believe this is really happening... it is overheating and because of that the system works bad... I heard Macs have some sort of heat management system and when the temperature is up the performance of the system decrease to save the hardware.... I played the same movie on my night blanky and on a cold glass surface and guess what? It works perfectly on glass and very bad on my warmy blanket...