I am making a game with over 300 rooms in a rather complex map that goes up, down and around and has sections organised as outer "rooms" encompassing the multiple inner rooms that the player actually visits. All is well for a while, but as I proceed through the game first a few visual oddities appear (strange faint shapes, as if the outer rooms are being drawn as strange polygons rather than rectangles, and/or random highly angled shapes are being drawn); then later, some rooms don't appear at all, possibly because they are drawn off really bizarre coordinates.

See the attachment. The top one is what you see having fallen down a hole then walked around a bit and down a slope: the player is in the middle of nowhere and instead of the rooms, there are weird black or grey shapes angling across the left of the screen. The display can be "repaired" by gently resizing or sometimes moving it: thus in the second shot, the player now appears in a partial room but others have now vanished; while in the bottom shot, the map has been fully repaired. Continuing to resize at this point allows the corruptions to reappear and disappear as you go.
In an even later section than shown, a large maze, none of the rooms are shown at all until you resize "correctly".

It is not the room design per se, as if you "jump in" directly by resetting the POV starting room, everything is drawn correctly.
Player movement also appears to slow down while this is happening.

So finally to the question: has anyone else seen this and does anyone know a solution?

I haven't seen this before. If you can send a link to your game so I can reproduce the problem, I'll take a look.

The game is still under development off line, but hoping to release it soon.