FPS drop when given approach vectors

Hi everyone. I have been having this persistent problem with FSX for a long time now. I normally get a very good frame rate in FSX. I have it locked to 20FPS (with the built-in frame limiter) and my PC has no trouble achieving that most of the time, resulting in very smooth game play. However, every time, and I mean EVERY TIME I approach an airport and ATC gives me the instructions "XXX, you are XX miles east, turn left heading XXX...."  my frame rate drops to about 1-2 FPS from the second the instruction pops up to about 15-20 seconds after ATC stops speaking. This problem is far from a serious one, but it is still irritating and really kills immersion. Any ideas on what is casuing this? Thanks!