Frame rate change between 2D and 3D cockpit

Hi everyone, has anbody any ideas why when I change from 3D cockpit veiw to 2D cockpit veiw my frame rates drops from 30 to 40 fps in 3D, to 12fps 2D, Below are my flight Sim PC build specs.  I use Rex Essential plus OD for weather and textutres.

Many thanks in advance for any answers

Dave

Operating system Windows 7 Pro 64bt

Processor Intel Core I7-980X Hexa Gulf town @4.2Ghz

P C Case Lian li PC-V1000 plus Aluminium case

Motherboard Asus P6T Delux V2 X58

Memory Mushkin 6GB PC3-12800 low Latency

Heatsink , Titan Fenrir Evo

SSD Drive OCZ Vertex 3 Windows 7 Professional OS

Hard Drive western Digital Raptor 300 GB Microsoft FSX

Hard Drive Western Digital Raptor 150 GB

Graphics Card EVGA GeForce GTX 580 1536MB GDDR5 OC

Sound Card Creative X-Fi Titanium Fatality Professional

Optical Drive Pioneer DVR116DBK

Blue Ray 4 BD ROM, LITE ON

Power Supply BFG ES 800 Watt

Saitek Pro Flight yoke & Throttle

Monitors

Dell U2711 Monitor

Dell 17” Flat Panel