will not recognize pH meter

[agorden]

I have two setups with Labview. Each runs a autoburet, a pH meter, and
a UV-Vis spectrophotometer. On one, every now and again when you use
the spectrophotmeter the pH meter gets "stuck" and will only read 0mv.
This can usually be corrected by turning the program off and then back
on. This is annoying but not critical. On the second setup, labview
will no longer recognize the pH meter at all, always reading 0mv
although the pH meter by itself works.
Any ideas?