Steering is stiff at slow speeds but not all the time

Typically that is caused by a weak power steering pump. Intermittent loss of power steering assist is most commonly caused by internal leakage inside the rack and pinion steering gear, but the symptoms will be different. You'll see a loss of assist in only one direction first, and it will usually return after a few minutes when the engine and power steering fluid warm up. Loss of assist will occur when turning the other way weeks or months later. The big clues here are it affects turning each way differently, and engine speed is not a factor.

When the pump is weak, due to worn vanes on the impeller that pushes the fluid, loss of power assist will always occur equally in both directions, and if it returns, it's when you raise engine speed. Fluid temperature doesn't really affect when the power assist returns.

If the pump is not the problem I have seen where the steering control module would intermittently fail. Here is the steering control module wiring and location

Please let us know what you find.

Cheers, Ken