I found the cause of the inifinite loop problem. It is an INFOPATH design fault problem.
The cause is, you cannot have more than somewhere about 9 consecutive calculation controls (ie fields).
I mean 1st field initialises,
2nd field uses first plus some extra.
3rd uses the 2nd plus edxtra. etc etc
until 9th uses the 8th plus extra data.
The infinite loop error then occurs.
The above construction had to occur that way because you are limited to conditions assignments in the text box properties.
If an 'else' structure could be set up in the text box, that would be nice too. I think, off the cuff, it would seem impossible to setup.
THEREFORE, AN URGENT REQUEST To Microsoft, to remedy this problem.
And that is, increase the number of calculations allowed, Somehow.
How I worked around it, was:----
I broke up the 9 controls (ie fields) linkage into 3 separate blocks, and joined the calculations in the last control.
Ian