The ports XBL needs are:
TCP/UDP 3704
TCP/UDP 88

The freezing is caused by Xbox live. I haven't played Crackdown, but the symptoms you are explaining in CoD 4 are normal, and suffered by much of the CoD 4 community. Once Microsoft buys more servers this will be a lot better. Also, you might try clearing your 360's memory cache. Go to the system tab in the xbox dashboard and click on memory. Press Y over the hard drive symbol, then press X, X, Left bumper, Right bumper, X, X. This effectively defrags the hard drive, and brings up a handy menu of random crap you never use that you may want to delete.