Update:
Tuesday couldn't get an appointment at the repair shop so I worked on other show elements.
Wednesday... 6 hours at the repair shop: "It's a software problem." Reformatted hard drive, reinstalled XP and drivers, ran sequence at shop for 1/2 hour no problem. Hooked up everything again at home, and within minutes "Access" denied. At one point the sequence did run for about 20 minutes before execution stopped.
Today am: sequence ran for 1 hour 48 minutes before Access error. Took PC/Dongle/Splitter/Lynx#2 back to shop. Sequence ran fine for over an hour. Figures, it wouldn't hiccup while I was there. I kept telling them my problem was a communication error. They plugged in a USB mouse and said "but USB works fine!" Further frustration when I asked the tech what he would do IF the error did occur, and couldn't get a clear answer. Bought a new PCI USB card. Tested sequence for a few minutes at the shop with Dongle plugged into new card... no problem. Came home and set everything up again.
I don't want to jinx myself, but various simple sequences have been running fine now for five hours with no Access errors.
John
[edit to fix days... I don't even know the date anymore!]