Dealing with Mech as Protoss

So, I have been having a lot of trouble dealing with late game Terrans who go heavy Mech. We're talking lots and lots of siege tanks, Thors, and air. Basically, I've found this composition pretty impossible to crack. If I go ground, the tanks blast through virtually all of my units before they can close. If I go air, the Thors and Vikings tear me apart. If I go Immortals, he mixes in Marines, Ravens, banshees, etc. And forget drops--by this point he has more turrets then you can shake a stick at. The whole thing is just incredibly frustrating. Has anyone had any success dealing with this?