Use less objects in one area, don't use lights, use less objects with glass in them etc. A picture of said area on your map would help a lot.
Add an aesthetic piece that you drive under that blocks out most of the visible track. That way, all the objects pop-in at once instead of individually.
Try using some rocks maybe as a LoS blocker. FR lag occurs when many objects are seen at once and is more potent when the objects have lights on/hitting them, have glass, and so on [like Overdoziz said].
Put the rocks (or whatever items you choose) near the wall double stretch of your track. That way, when you're driving the rest of the track, those pieces are blocking your view of those bad FR pieces.
It is such a pity I don't have any of the DLC for Reach, because that race track looks aesthetically brilliant...
I think the double doorways have lights that may be causing problem for you. They are quiet a detailed piece... The curves of your track look great. I think because you have alot of pieces showing as you race in view it could be causing. Maybe structure to block lines of sight in race so you only see sections at a time, although adding more may cause problems. Are you at max objects / full budget? I think as ridge line already has a lot to render of original map it struggles with more added. I started forging on ridge line ...noticed fr problems and gave in straight away.