Weird frame rate problem in FS2004

Overall I think FS2004 is an improvement over FS2002 and am satisified but I have an wierd problem with the frame rate. For example, I take off from DFW at 26 fps on the runway and during the approach to DFW inline with the runway I get around 12 fps, but here's the wierd part, I restart a flight from DFW and slew for an approach where I had only 12 fps but this time it's 20 fps. Landing on the same runway and same display settings as when I had 12 fps. Another example is slewing around Los Angeles. I get the same frame rate problem when slewing around Los Angeles and Long Beach. Try this, slew from LAX then to downtown LA then slew to the south of Long Beach Airport, turn to face north with downtown LA in view and check the frame rate, then increase the autogen and then check the frame rate for an increase. Then restart a flight from LAX and slew directly to Long Beach at the same spot as before and notice that the frame rate is faster than when you slewed to downtown LAX before going to Long Beach. Another wierd thing is that when the frame rate dips to 15 fps then I increase autogen and the frame rate goes up to 23 fps taxing at LAX after slewing to downtown. Tried my Ti4600 and still the same problem. Is this because the autogen that is not visible anymore because I'm too far is still in memory and being processed by the CPU and when I change the autogen setting it only puts the autogen objects that are visible in memory? One good thing I noticed is that I hardly get a peformance hit in FS2004 when 4x FSAA and 8x AF are turned on my for my Ti4600 whereas in FS2002 the peformance hit was too much with those features turned on on this card.