Ive never had one go that way. Always simply faded when the fluid and tires got hot in combination. Could raise the engine rpm and would get the power steering back. Sometimes changing the fluid helped and sometimes it didnt.
I've seen the p/s pulley shaft break internally and it will look fine but, its in reality not doing anything. Pull the belt and then pull on the pulley, if it comes out, put a pump, if it doesn't budge, put a steering rack on it.
Its probably the rack, they suck on this vintage Chevy. I replaced my rack at 70k miles, it was done. Now at almost 90k its leaking again, great.
If its any consolation, GM improved them on the 2011's.
My girlfriend's PT Cruiser had the same problem. Steering working fine one second, then no power assist the next. The dealer replaced the rack and that didn't fix it. Then they replaced the pump and that fixed it.
If its not a broken shaft it could be (probably) the pressure control solonoid (2).
Edit: Apparently you can do a easy EVO delete on this unit by using GM part# 26009894 fitting and Gates 353950 PS pressure line.
In any case do not toss the pump for a rebuilt unit, fix the original.