Are the fonts that are reflowing resident on the macs themselves? If so are absolutely 100% sure that they are identical, I don't just mean called the same?
Here's an easy way to track down the greif if you have a server...
• first of all clear all you font caches
• clear out any fonts from Library and User Library you absoluetly dont need, see this link -
http://www.prepressforums.com/ftopict-5602.html
• Then copy the font from the 'good machine' to a server.
• On the bad machine load the font into whatever flavour of Suitcase you're using and activate it
• Open the offending doc and watch for reflow, in theory it shouldn't happen, if it does after following these steps post back and we'll take it from there.
(it's 5.30 am on Saturday in the UK but my machine will be on all day, certainly for the next 14 hours, and I'll be infront of it for most of that time)