Yup, I've seen it too.  Ironically, I didn't start seeing it until I upgraded my box from 1GB of memory to 2GBs.  I think I just got a little more 'liberal' with my Visual Studio instances after that.

I see it many 2 -3 times a month.

I am not sure that it is VS (or at least the IDE part).  We have some internal tools that call msbuild directly and I've seen that before on occasion.  I've only observed this issue occur when trying to compile projects with a lot of references so I wonder if that has something to do with it.

I periodically delete my solutions .SUO "prefs" file, as when this grows too large (on web projects, for e.g.) it can cause VS to use huge amounts of RAM and finally die.

I had this recently. It was an error in Visual Studio, but it originated from MSBuild whilst trying to compile. A restart of VS solved the problem though.

(1) Locate [HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings\User Agent\Post Platform]

(2) Delete "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1) "=""

Jonathan,  your solution works! Thanks!