occasional very slow analysis time

I find that most of the time, clang 2.7 analyzes a .c source file
quite quickly (under 0.2 user CPU seconds). But occasionally
it will take a very long time (5.0 user CPU seconds).
The "slow" source files are both .c and both include approximately
the same .h files. The slow files are always the same -- repeating
the run gives the same results. Any suggestions on diagnostics I could turn on
so that clang would dump intermediate times for each pass / phase
to help figure out where the time is going?
[regrettably I can't post the actual source code yet]