Mircosoft C V5.1 floating point problem.

[H Bruce]

I am working on a large program that has suddenly starting crashing with
the following error:

floating pointing exection 
M6101 invalid

I tried to track the error down by using CodeView but compiling the program to
prepare it for use by CodeView stops it crashing.
After the program has been run and crashed a few times it starts to work and
will not crash until the PC is rebooted.

It will be alot of work (Edit - Compile - Run cycles) 
to insert printf() statements to track down the problem especially if it
periodically disappears.  So to give me a head start .....
does anyone know what is likely to cause this crash ?

The manual says it is caused by operating on a NAN (not a number !) which is
not much help. I have carefully casted all integer / float operations
and checked parameter passing.
The compiler gives me no warnings that are of use.

Getting desparate.....

Henry.