In LabVIEW versions up to 5.1, the VISA resource name for COM1 on a PC is "ASRL1::INSTR", for COM2 "ASRL2::INSTR" etc. I believe LabVIEW 6 and later allow you to use the COM names as aliases but I'm a late adopter ;-) In any case I believe the ASRL notation should still work (someone else can probably help here). However, if you say your program communicates OK to start with then the resource name you are using is correct.
If you're seeing the error you mentioned after some time, I wonder if you are opening lots of VISA sessions but not closing them again, so eventually VISA runs out of resources? Generally you should open a VISA session in your program before you start communicating, wire that session through all the VISA VI's you use, and only close the session when your program finishes. If you're seeing some other intermittent error such as a timeout, consider timing issues - maybe your instrument sometimes takes slightly longer to respond, or you try to read data while the instrument is only half-way through sending it, etc. You may need to think about how to make your program more robust against these problems.
