Power steering pressure too high??

[signal15]

The power steering on my mitsu is always hard to turn at idle, which indicates that maybe the pump is going bad. But, it makes a whining sound when turning it hard. That whining sound is the pressure relief valve.

If I flush out the system with new fluid, everything works fine for a couple of months, and then the problem slowly comes back. Unless it gets cold... then the problem is back with a vengeance. Last year, it popped off one of the hoses under hood and sprayed fluid everywhere. I put it back on and double hose clamped it. Yesterday, it appears another hose underneath either burst or popped off when I turned the wheel hard leaving the driveway.

Thoughts? What would cause the pressure to get so high the hoses are bursting or popping off?

The weird thing is, replacing some or all of the fluid tends to make things a lot better for awhile.