I use MT6 on every flight set at 15% and have zero stutters in both 2.5 and V3.

 

Do you have the settings at 15/15 or 15 for both combined?

Do you have the settings at 15/15 or 15 for both combined?

 

15% for airline only. 

Just flew a short hop from LOWW to LSZH in the PMDG 737...zero MT6 airline traffic and I can confirm I had much smoother (albeit much lonelier) flight experience. Gonna try and creep this up but previously at only 10% airline and GA traffic I was getting more erratic FPS and frame stutters even at cruise altitudes. 

I'll add another vote for UT2 and in my experience it's much better on FPS and smoothness in P3D for me.

 

In fact, it wasn't even close IMO.

What happens if you change the texture resolution to 1m? (Instead of 7cm)

 

This is what I've found I need to do in both FSX and P3D . I thought it was just my older 4ghz 2500K but maybe it will help you