Escalation graphics - elements missing, showing as black [Solved]

Many small parts of objects are missing and therefore appear as pure black. This includes top rotating part of Nexus; rotating part of Constable (except the lights appear); base of Smarties; several other things. Windows sometimes reports low memory when starting the game but there's not a direct correlation with that and the graphics glitch.  This glitch at first appeared intermittently but now always happens.  It does not occur in the base game Ashes.