I have a 5 year old Arcteryx Gamma SV made with powershield that I like a lot, but it's getting pretty worn out. The current Gamma SV is now made with a fabric that isn't quite as warm.

I'm tall and skinny, so it has to be slim cut. Most patagonia and mountain hardwear stuff is too baggy.

Any ideas?

The deadbird GammaMX is pretty slim in fit, even for Arcteryx. It is also a bit warmer that some soft shells, for example the Patagonia Guide Jacket.

Have you checked the Eddie Bauer/First Ascent options? I'm 6'5", 200lb and find a lot of FA garments in Large/Tall that fit just about perfectly.