I have logged 3 speeds of 656Kbps download, 86Kbps upload.
The three software will vary your connection based upon your usage, so when browsing web pages your not really requiring the full bandwidth so it will revert to 3g.
Your highest download speed seems low - mine has peaked today at 3584kbps download.
a good place to test any speeds is
Speedtest.net - The Global Broadband Speed Test
I ran this yesterday and was getting 2mbps download 256k up.