Running on gtx 550 ti and i3, everything ran just fine on all max settings (except for antialiasing) until I've decided to install enb.

I started the game and loaded a save in whiterun (outside), I started running around and had a stable 60 fps but in some places when I move the mouse and look around, a massive stutter starts, its unplayable, the game literally stops for 5 to 10 seconds and it was obvious to me that the machine is struggling with it.

The weird part is that after playing around with enblocal and changing some values, the stutter goes completely gone if I enable AutodetectVideoMemorySize.
The other weird thing is that while I have better fps but heavy stutter with a predefined memorysize (either 256, 512 or 1024) and autodetect off, I've no stutter at all but half as less fps if autodetectmemorysize is enabled instead.

Does anyone know the reason for this?

You running out of video memory when memory manager is enabled (ENBoost) in enblocal.ini. Disable it or tweak it (or ask memory setting from users with gf5xx videocard of same vram size, but it rather not helps, because of not same mods installed, game is very individual).

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i5-4690k, 16Gb RAM, GTX 1060 6Gb, X-Fi Titanium, Win7 x128
I am INFP, not the brutal, godamnit.