Orthomode keeps switching itself OFF. Anyone come across this before?Stephen

It's not turning itself off.  I suspect it would be more accurate to say something keeps switching Orthomode off.  You probably have a custom routine or command macro that turns it off for its own internal purposes, but does not turn it back on.  This is especially likely if you have custom commands that you might sometimes get out of using ESCape, which may have the resetting of Ortho built into them at the end, but perhaps don't have error handling to ensure that it gets reset even if you cancel it rather than let it run its course.If you use any command names that are not native AutoCAD commands, or command macros in toolbar buttons or other menu items, or Scripts, and can investigate the code that defines them, search for ORTHO anywhere in them.  It could be just something like  ORTHO OFF  in a macro, or (setvar 'orthomode 0) or perhaps (command "ORTHO" "OFF") in an AutoLisp routine.  There are variant ways to do it [for example, that last example could be done (command "_.ortho" "_off")], but looking for the ORTHO part [in any case combination] should find anything that would be doing this.

Kent,Many thanks for your very full response. Unfortunately, you've ascribed way more capacity to my AutoCad facilities than I actually possess. I have no custom routines or command macros and I only use ACAD commands. I'm using 2016 LT running on a straightforward Windows 64 bit platform and I've never had this before. Not that this is the only weird thing that's happening. The hatch command has suddenly started throwing a wobbly and producing super dense hatches which I then have to scale back. I've been using ACAD LT for many many years and am quite used to it going a bit rogue from time to time - but these two items are really getting to be annoying now.

Did you install the 2016 service pack?You can also try Start > All Programs > AutoDesk > AutoCAD 2016 LT > Reset Settings to Default.
John Connor2016-01-15 18:33:58

Thank you John. Didn't know there was a 2016 Service pack for something that had only relatively recently been issued. Tried looking for "Reset Settings to Default" in the path you gave but all I had available when I right clicked was 'Restore Previous Versions' - not quite the same thing.S

Where did I mention right-clicking?  The Reset settings option is via the Windows Start button.  Why don't we just search on the topic?  And here we go.....https://knowledge.autodesk.com/support/autocad-lt/learn-explore/caas/CloudHelp/cloudhelp/2016/ENU/AutoCAD-LT/files/GUID-113BB319-8D7B-4A44-ABC2-DFE56C26C680-htm.html
John Connor2016-01-16 13:00:46

Thanks John; apologies - hadn't looked down the list before going into auto-drive and right clicking. Stephen