Gpib write or read sometimes hogs cpu

Question

I have written a simple vi to communicate and
control a Stanford Research SR640 programmable filter.  It does a
series of gpib writes and reads to get and set various controls on the
instrument, and it works very well.  The problem comes when I try to
call it from a more complex vi that controls the entire experiment. 
Once the sr640 vi is linked into the big vi, it takes 10-20 seconds for
the sr640 vi to return and it hogs the cpu, making even the mouse
unresponsive, Windows task manager stops updating, etc.  This happens
even when running the simple sr640 vi directly from its front panel
with no other vi's executing.  In "lightbulb" mode, I can see that
occassionally, the gpib write or read icons (from traditional gpib
pallette) take a long time to return, and the cpu goes to 100%.
If
I remove the complex calling vi from memory, the problem goes away, ie
the sr640 vi runs fine.  If I load the big vi back into memory, I
reproducibly get the bad behavior when running the sr640 vi. Note that the big vi does not need to be running to cause trouble, only be loaded in memory.   If I
delete the sr640 vi from the big vi, i.e. it is no longer called from
the big vi, the sr640 vi again runs fine.
Any idea what might be happening and how to go about debugging this?
Thanks,
Bob Shelby

Answer 1

Thanks for the reply. 
Attached is the SR640 vi and a calling vi that works just fine.
Bob 
Attachments:
srs_filter_setparms.vi ‏42 KB
test_sr640.vi ‏17 KB