dslr - Why does using IS with my 70-200/2.8 lens remove detail?

Can someone explain why my 70-200mm 2.8 is is ruining photos with IS?

I'll shoot at a shutter speed of 200-250, at around 160mm.

When I have no IS, all the pores and wrinkles are perfectly sharp. If I turn it on, it seems to smooth everything out.

I can't imagine why.

Is IS for slower shutter speeds than 1/250s?