Wish I could help but all I can do is corroborate - I experience
similar things at work.

No new OS's (still on XP), Office versions or other software (that I
can detect). It started in Office 2007 approximately mid-April 2011,
and that is after using 2007 for more than a year on the same machine.
Doesn't matter whether I'm on the network or completely isolated, have
a single file open or a dozen apps and files open.

Tools that have worked perfectly for years by multiple employees have
begun 'acting up' or stopped working completely due to:
1. Lines of code that are skipped or only partially executed before
going to the next line.
2. Functions that have ceased to be recognized, i.e.
(Environ("blah"))? "I don't know wtf you're talking about, here's an
error!" (This really throws a monkey wrench into automated file and
folder control for multiple users in a corporate setting.)
Various other errors, but these two are the ones that have cost me the
most time, energy and focus.

I have worked on some of these files with other employees around the
country with similar results. I have tried on every company machine I
could get my hands on, none acted better and most acted worse. In each
case, the code executes perfectly EVERY TIME on my six-year old home
machine (XP/2007) with half the processing capacity and 1/3 the memory
of my company machine. Upgraded to Office 2010 at work after first
experiencing these things and results are the same.

Seems like a resource allocation thing - like the machine doesn't want
to execute a line, or decides part-way through it doesn't want to
finish executing a line and just goes on to the next line and repeats
it's finicky little act. I cannot consistently reproduce, and it's not
always the same things that break.

Any IT folks out there care to share or speculate? Restarting apps/
machines does not help. Hard drive space plentiful. Temp files
regularly cleaned. Defrag occasionally. Issues occur in the same
projects whether in fresh files or 10-year old files. Most of the
tools are considerably less than 100kb, the absolute largest is less
than 6mb fully loaded. Did I miss anything obvious?