I don't see this behavior with CS5 under OSX 10.7.4.

Are you running 7.0.4.553? (Hold down Control and select About InDesign)

Have you tried FAQ: How do I reset or "trash" preferences?

I assume your zero point is set to the left margin, i.e. 3p4 relative to the spine?

You haven't mentioned either your desired column width or your gutter size for the 4C grid, so it's a bit tough to be certain of what you wanted. I assume you wanted a 4-column grid with 5p gutters, so your first grid line should be at 11p1 (relative to the left margin)? Because that's what I get.

And the far left and far right guides are right at 0p0 and 59p4.

InDesign internally uses points for its calculations (and floating-point decimal fractions thereof), but your problems sound like round-off errors. Yet they are far too large to be round-off errors...

Have you tried recreating your document from scratch?

On another machine?

As a last resort, you can export to IDML, unzip the file, and look at the actual coordinates of the margins and guides (this will be a pain).

That was CS2 running on Intel Macs under rosetta.

Bob

I thought I was as old you.

Bob

No, but I am deciding on an early pension.

Bob

Ah, you had said a 4-column grid, but you meant a 5-column grid. Gotcha.

Please try recreating from scratch. It's essentially what you've asked the rest of us to do (and what I did), so you should take the time to do it as well.

7.0.4 (and 7.0.3) fixed innumerable issues, not all of them in the ChangeLog. The benefit is huge, you should push for it. What version are you using, then? 7.0.0? You did not specify.

If I change the guide grid to 5 columns, things are fine.

Oh yes, originally you said:

Because of the way our ad software works, standard columns have to be set at 6 columns with p8 gutters. We make these columns white, so they look invisible on the page.

What do you mean by making the columns white? Do you mean you set the column guide color to be white instead of maroon? Or something else?