I have a SP2010 farm that works perfectly internal. The farm has 3 web apps on different ports, the one being naughty is on port :82 internally and it internal address looks like this: http://SharePoint:82
I publish this site using TMG 2010 with an external URL of http://portal.westhatch.essex.sch.uk
All seems to work fine until you try to access any list items or forms and I then get strange messages about the list does not exist or the form URL adds the :82 to the end of the external URL
I am seriously running out of ideas at the moment.