Michael,
Your instrument is hanging for some reason. It won't even respond to the FindLstn command when it's in this "hung state", and the only way to fix everything is to reboot the instrument. I think it's time to bring Keithley into it and see if they have any thoughts on why this might be happening.
One longshot that will be a change and may affect the timing of everything just enough to fix whatever the root cause is...try changing all your writes & reads to synchronous instead of asynchronous. I haven't tried this in LV 5.1, but you should be able to right-click the GPIB write and GPIB Read VI's at the lowest level and hit "change to synchronous". If this fixes the problem, I'll still suspect that your instrument is doing something funny but it may at least get you up and running.
Scott B.
GPIB Software
National Instruments
