PDANET
I would suggest you remove any drivers you have installed from Windows 7.
I was having the same problem and then heard about PDANET.
I couldn't even use fastboot because everytime I connected my Streak, Windows would try to install different drivers.
I read in another forum, pocketables I think, that installing PDANET would solve this.
So I uninstalled all traces of drivers and the Dell Sync also, installed PDANET and voila, no more problems.
So if PDANET isn't working for ya, you have a driver issue or there's something wrong with your Streak