Ah, this sounds just like what my Deluo GPS unit is doing. It too is a USB unit.
Their support team had me download a port replicator to make it work. It did, but was a pain to keep using, and I still got crashes where S&T lost seeing the antenna.
This GPS unit worked on all the early versions of S&T, but to 2006. With 2006 forward I had nothing but trouble with the Deluo unit. It is a nice waterproof unit with a strong magnet, and a long cord.
My solution was to buy Streets & Trips 2008 with the Microsoft GPS antenna, which has a USB connector.
But it works, every time. Just the cord is too short for my liking.