This flicker problem actually a directX driver problem. It can be triggered by switching the active windows, starting addons like SquawkBox or (most common in my case) switching from full screen to windowed mode. Sometimes i have this bug. Never in DX10 mode. I use GTX470 card. drivers later than ver 285 to ver 301 had extremely bad reputation. Now im on 306.23 WHQL. Pretty stable.
It happens with default birds too, but not so often.
It can be one of FSX hidden "feature".

Another apearance of this crap - is when your VC just becomes transparent. - its same problem.