What causes a plane to snap ? I have my elevator throws set below manufacturer specs to keep the plane from snapping. Is it a cg issue?

The CG may have some effect, but not that much. Describe the plane and how many degrees of throw you have on the elevator.

some planes like the cap has to be a bit nose heavy and have some speed on landing to stop the snap from happening. Warbirds also have the same issues. If its snaping while inverted role its usually CG if it snaps when landing its usually lack of speed on approach.

Specs
Wing Span: 72"
Weight: 11.01lb
Length: 66.5"
Wing Area: 992sq.in
Wing Loading: 25.66oz./sq.ft.
Engine Required: 2c 108 ~ 160, 4c 120 ~ 140
Radio Required: 4 Channel 6 Servos

Cmp Katana 140 w Saito 180, added 13oz to the nose for balance, weighs 13.5 lbs

I have a 1/2 inch in the elevator for throw, any more than this and it will snap. From level flight it looks like a full stall, while in a loop it just just plane stalls. falls out of the loop hard and fast

Hi!
Sorry! But your model is way to heavy!
That what's make it snap roll...wing loading.5 kg (11 lbs)is quiet axceptable ...13 pounds isn't .
But you can make it lighter.

Too heavy. Your wing loading is over 31oz/sq foot. If you have rear mounted servos, try moving them forward to the cabin area or wherever you have room, and remove some of that nose ballast. Good luck!

I moved all the servos to the center of the plane, I am using a kevlar pull pull on the rudder. Before i moved the servos from the rear of the plane it took a full POUND to balance the plane. I even have the battery mounted to the motor box.

Something is wrong with the weight. The plane was designed for a 2cycle 108 - 160. So the Saito 180 is probably plenty big up front. You should not need any additional nose weight. Have someone help you check the CG again.