Power steering problem

[Mustela]

Ok so some days it seems fine, then next time I start the truck I have no power steering but it comes and goes if I rev the engine. Fluid is full but acts like I have no pressure. And it sometimes feels loose on the highway. So y'all think its in the speed sensor or ps solenoid or can it be the pump? I figured if the pump went out, it would just quit, not come and go....thx

 

[SunlitComet]

sounds more like the steering angle sensor but the system is scannable with the right hard/software.