RE: Nav computer problems
Try disconnecting the PC connection to the "system" before booting up, while leaving all other connections as normal. Once PC is up and running (but you won't have a GPS indication on the Nobeltec), and all the other goodies are still operating, plug the connection into the PC and see what happens. If the "system" dies when you do this, it's probably the serial card.
Also, you can buy/fabricate what's called a "loopback" cable. The connects the Xmt & Rcv together on the back of the PC. You can then run Hyperterminal, create a "new" terminal connection, and whatever you type on the screen should echo back at you. Again, if it doesn't, it's probably the serial card. If it does echo, check the cable from the PC to the terminal block.
Hope this helps.