Why does my internet connection fluctuate so much?

I pay for internet speeds of 6Mb down and 1Mb up, and when I do a speed test online my download speed will fluctuate from about 5.8Mb to 6.3Mb which is more than what I'm paying for. Also when I do a ping test my ping will fluctuate from about 40ms to 50ms and my jitter is usually pretty high. So does this mean I have an unstable connection, and if so how do I fix it? And please don't suggest that I switch isp's cause this is the only one available in my area.