I'm betting is video card related. Especially after you said using the default nVidia settings made it worse.
Is the GTX 750 new or did that come from the old system? It's a little lower powered but it should manage IL-2 1946 no problem (you say it does BoS just fine anyways
).
In the nVidia control panel setup a custom set for IL-2 1946.exe.
- Turn off Antialiasing Transparency
- Set Power Management to "Prefer maximum performance"
- Set Threaded optimization to Off.
See if that helps.
Basically IL-2 is a very old game using OpenGL (even in Direct3D mode - turn that off BTW... go back to OpenGL), and it doesn't understand or work with some of the optimization and power controls that modern GPUs have. It has a terrible time with some of the modern antialiasing too. Go full out 8x AA but don't do anti aliasing transparency (that'll choke on clouds and fences in IL-2 for example) or anything fancy.