The instruction address was constant. I observed several values of X on separate invocations of dot, including:

0x3420324b
0x3720394f
0x3420324b (2nd time)
0x32203329 *** see stats
0x342c384e
0x36342051 ****** see sample input for this invocation
0x2c333448

Some stats upon report of ***
- Mem Usage: 420912K / 766852K
- Disk drive free space C: 4 GB, D: 8 GB
- Dot.exe Mem Usage: 3796 K, VM Size: 3212 K, Handles: 12, Threads: 1

Running DOT from Doxygen v 1.2.8.1 with the following options:

#---------------------------------------------------------------------------
# Configuration options related to the dot tool
#---------------------------------------------------------------------------

# If you set the HAVE_DOT tag to YES then doxygen will assume the dot tool is
# available from the path. This tool is part of Graphviz, a graph visualization
# toolkit from AT&T and Lucent Bell Labs. The other options in this section
# have no effect if this option is set to NO (the default)

HAVE_DOT               = YES

# If the CLASS_GRAPH and HAVE_DOT tags are set to YES then doxygen
# will generate a graph for each documented class showing the direct and
# indirect inheritance relations. Setting this tag to YES will force the
# the CLASS_DIAGRAMS tag to NO.

CLASS_GRAPH            = YES

# If the COLLABORATION_GRAPH and HAVE_DOT tags are set to YES then doxygen
# will generate a graph for each documented class showing the direct and
# indirect implementation dependencies (inheritance, containment, and
# class references variables) of the class with other documented classes.

COLLABORATION_GRAPH    = YES

# If the ENABLE_PREPROCESSING, SEARCH_INCLUDES, INCLUDE_GRAPH, and HAVE_DOT
# tags are set to YES then doxygen will generate a graph for each documented
# file showing the direct and indirect include dependencies of the file with
# other documented files.

INCLUDE_GRAPH          = YES

# If the ENABLE_PREPROCESSING, SEARCH_INCLUDES, INCLUDED_BY_GRAPH, and
# HAVE_DOT tags are set to YES then doxygen will generate a graph for each
# documented header file showing the documented files that directly or
# indirectly include this file.

INCLUDED_BY_GRAPH      = YES

# If the GRAPHICAL_HIERARCHY and HAVE_DOT tags are set to YES then doxygen
# will graphical hierarchy of all classes instead of a textual one.

GRAPHICAL_HIERARCHY    = YES

# The tag DOT_PATH can be used to specify the path where the dot tool can be
# found. If left blank, it is assumed the dot tool can be found on the path.

DOT_PATH               = D:DevToolsGraphVizGraphVizBin

# The MAX_DOT_GRAPH_WIDTH tag can be used to set the maximum allowed width
# (in pixels) of the graphs generated by dot. If a graph becomes larger than
# this value, doxygen will try to truncate the graph, so that it fits within
# the specified constraint. Beware that most browsers cannot cope with very
# large images.

MAX_DOT_GRAPH_WIDTH    = 1024

# The MAX_DOT_GRAPH_HEIGHT tag can be used to set the maximum allows height
# (in pixels) of the graphs generated by dot. If a graph becomes larger than
# this value, doxygen will try to truncate the graph, so that it fits within
# the specified constraint. Beware that most browsers cannot cope with very
# large images.

MAX_DOT_GRAPH_HEIGHT   = 1024

# If the GENERATE_LEGEND tag is set to YES (the default) Doxygen will
# generate a legend page explaining the meaning of the various boxes and
# arrows in the dot generated graphs.

GENERATE_LEGEND        = YES

# If the DOT_CLEANUP tag is set to YES (the default) Doxygen will
# remove the intermedate dot files that are used to generate
# the various graphs.

DOT_CLEANUP            = YES