SSR Fail?

So my system is a 5500W heating element mounted in a keg controlled by a PWM that I built according to Walkers design. Current is controlled through a 40A SSR but something strange keeps happening with it. Every time I go to heat water the PWM works great and I can adjust the heat perfectly well using the knob and see how often it is clicking on the SSR because of the built in LED. However after about 30 sec of full power I lose the ability to control the heating element. The LED still pulses according to where I have the Pot set but the element is at full power. Has my SSR failed? That is the only reasonable conclusion I can come to.

How are you testing that the element is at full power?

I can hear how hard it is running in the water when it is turned all the way up and I can hear it pulse when I adjust the potentiometer. However when I leave it on high for more than 10 seconds it get stuck full throttle and I lose the ability to adjust.