I'm sure you have an attenuator on your system because a 200mw PC Card connected to a 24dBi antenna would result in 50W of total power, not accounting for cable and connector loss.
With that said, given your scenario, RF power is not the problem. If anything, you have too much, but my theory with RF and cars is the same. There is no such thing as too much power, although the FCC does have something to say about the RF part. You should be getting a total of about 3 to 5 Mbps on that link given that you are running 802.11b. Are you talking about getting 1Mbps up and down to the Internet? And if so, is the Internet the bottleneck?
I don't have an absolute fix for your problem, but a 24dBi antenna at only one mile would take a bit precision to aim. Not that it is hard, but you had better have it mounted solid. If you are just holding the antenna and aiming by sight, then that could be a problem.
Let us know a bit more detail and see what happens!