2003 GMC Sierra power steering

My truck is a 2003 GMC 3/4 ton Duramax Diesel 4x4. After driving the truck for 2 or more hours at 70 mph or more, the power steering starts making a whining noise. After 4 hours, the fluid starts boiling over from the reservoir. This happened right after I had the system flushed. The pump and a hose were replaced, but the problem persists. Previously, there was also a whining noise which we thought was coming from the steering column, but has stopped since this new problem surfaced. Transmission fluid was used in power steering when it initially boiled over, and the system has been flushed again since the pump was replaced. Then generic power steering fluid was used after a subsequent boil over.