[Mikey]

interesting comment you made that it feels fine up to 65mph but then vague after that.. is vague = non-responsive play in the system or does it mean the steering feels too "light" or too easy to turn?

one culprit could be in the rebuilt p/s pump and rack, if the flow control valve is of the wrong size (this valve is easily changed with the pump out of the car and should be selected to match the requirements of the R&P in the car.)
in this case, the steering would feel too "light" and you can confirm by going 75mph on a straight section of highway and turning the wheel only slightly to see if the car responds (even though it may feel vague or disconnected at the steering wheel).

in the garage, there should be virtually zero play in the steering wheel. on my car for example, I can turn the wheel 5-10 degrees max, with some slight resistance, while the engine is off. After that it becomes much harder to turn. Note the initial 5 degrees is not actually "play", it is just compliance in the system that is easy to overcome such as tire flex, rag joint, etc. If I watch the tire closely it starts moving IMMEDIATELY in conjunction with the steering wheel, even as I go through the 5-10 degree initial turning.

subsequently, any slight movement of the steering wheel while driving does result in response from the car.

on the other hand, if your issue is with lack of control, as in the car may go right or left on its own even while you maintain a steady 0 degree steering input, if you are sure it is not being induced by grooves or ruts in the road, then suspect ball joints or rod ends. these can be checked in your driveway, simply raise the tire off the ground and then grab it from the left and right side and attempt to move it, and again at top and bottom.

report back on your findings.

Cheers,
Mikey