too much DNA on agarose gel - (Jan/30/2013 )

I have also recently experienced this problem. The DNA shows higher mobility when loaded in great excess. I had cut a vector of 7 kb to release 5 and 2 kb. when I load 5microgram on the gel, all I see is a big, smeary band with mean size 2kb. The same preparation, when run at 10ng, shows the two expected bands clearly.