Horrible bluffering

I bought a used MacBook Pro (2006) on Christmas of 2012. Nowadays I have really bad lags and bluffering. I go on YouTube and even the commercials are bluffering. When I watch videos I use auto for the quality but sometimes if it gets annoying I try to use 720p or 1080p but it takes forever. When I go Viki, a site to watch dramas, I try to watch them in HD and it never plays smoothly. I'm just wondering what I should do to stop with these lags or what have you. Also I don't download a lot of things, I only have a few pictures here and there. I usually watch videos and blog on my laptop.