Frame rate drops suddenly

I fixed one problem now I have another. I found out that the mesh I've been using since 2004 started causing my flights to crash with mostly a terrain.dll error although I did get others. I had went to the website that talked about dll errors. I can't remember the site name, but it said some was caused by corrupted mesh. So I removed the USA mesh I had it in one folder and did a few flights with no crashes, even with pmdg 744 going into fsdt kjfk. I had been using this mesh as I said since 2004. I think I'll wait and get fsgenesis bundle for fs9. But now, and it has done this before, I'll be flying along usually in one of my add-on aircraft, pmdg 744, 737s, pss 752, 777, level-d 767 feelthere 737 and out of the blue the frame rate will drop from (locked at 25fps) down to 1 or 2 fps. It will stay that way for anywhere from 20 to 100 miles, the go back to normal. This morning I was flying out of kjfk 31L, and after t/o it started and lasted for about 50 miles, I know people will say it's the NY area. Yesterday I was going from klas to katl. About 150 out of klas. desert no cities around at fl370 it did the same thing for about 100 miles, then back to normal. Then on decent into katl it did it about 50 miles out and back to normal at about 20 miles and stayed perfect the rest of the flight even turning on downwind looking at the airport. Airports or clouds or ai don't seem to have any affect. It just does it out of the blue. I don't even have anything maxed either.