At a certain height, all units dissapear, and reappear when you move around or some some more out/in.

My vid-card: ATI HD4670 1 GB
unit LOD distance = 600 (max)
unit icon distance = 1000

i think since the last update this started

It looks like this: (you still see the enemyspotter widget at work)
- and you still see the buildings being build

it was ever like this for me (ati 9700 and ati 4290), there is a certain distance were the unit switch to the icons and at a certain point i saw nothing

Seems like an ATI issue again..
I got it as well,using a 4850..

sounds like far textures are not working for you (thats what is shown between LOD distance and icon distance. i recommend not using it for now (eg, set icon distance equal to or lower then LOD distance). for me, a useful icon distance is 200, so i can't imagine you need anything above 600.

I think a very high setting will solve your problem. These are my settings:

UnitIconDist=10000
UnitLodDist=1000

Start spring and click "Edit settings" in case springsettings will not allow you to increase the value to more than to 600.

Same thing happens to me on a geforce

Those settings wont fix it, I tried both:
UnitIconDist=10000
UnitLodDist=1000
and
UnitIconDist=500
UnitLodDist=600

(and even UnitIconDist=500 will show me icons instead of units when I zoom out far, which i dont want, and the problem still remains below 500)

Oh and it only happens when you are spectator

used to happen to me when i had set settings too high

I think thats not it because I have it running at lowest settings

Oh sometimes it also happens when i'm playing, but far less frequent

i still think its something performence related, spec see whole map in the end. had same problem but it was long time ago and i got no idea what did i change in settings to fix it