I can confirm the memory leak, ran in to this problem with flythrough as well (somewhere last summer).
Both on windows 7 and on OSX.
It was very annoying, meant you had to 'babysit' the workstations all day, rendering only a few frames at a time, closing ArchiCAD entirely to clear up memory and then rendering the next few frames.
Best bet is to render to PNG for the frames and combine them all to one movie afterwards.
We have nowhere near the amount of RAM as you do (16 gb and 32 gb), but also a fairly sizeable building and sloping mesh.
To keep things reasonable, I tweaked the settings to the lowest acceptable quality, which meant it was rendering about a frame per 2-3 minutes.
Erwin Edel, Project Lead, Leloup Architecten
www.leloup.nlArchiCAD 9-26NED FULL
Windows 10 Pro
Adobe Design Premium CS5