Itoo have that happen what this means is your hard drive is getting too full, when it happened to me Iwas using a 250hdd and was 75% full of data.
two options I found that worked either delete games or programs not in use or pickup a second hdd for storage purposes.
I gues third would be mor ram.
hop this helps for you try it and see.
FSX Win7 Ultimate 64bit
Antec 9002 case
Acer S243HL 24" Led 1920x1080
Intel Core i7 860 Quad-Core LGA1156, 2.80Ghz
Corsair Cooling Hydro Series H50 CPU Cooler
ADATA XPG Gaming Series 8GB (4x2GB) DDR3 1600
ATIXFXHD 6870 1024MB GDDR5