The power assist on my power steering is intermittent

That is typical of worn seals in the rack and pinion assembly although when you observed that increasing engine speed brings the power assist back, that is how weak pumps often act. If you always have power assist when engine speed is higher than idle, suspect the pump. The only way to be sure is to use a special tool. It is a valve and gauge assembly that is inserted in the high pressure hose between the pump and steering gear. With the engine running and the problem occurring, the valve is momentarily closed. If the pressure reading comes up real high, the pump is working and the rack needs to be replaced. If the pressure remains low, the pump is worn and must be replaced.

Try bleeding the steering pump and see what happens if it continues have the rack n pinion further checked-

Your problem looks like is going to be a faulty power steering moduel. Have the shop look up tech journal #18036 Here is a description of the tech journal.

DESCRIPTION:
Customers may complain of intermittent low power steering assistance on vehicles with speed sensitive steering. After cycling ignition key, the steering assistance may then be normal. This does not affect the hydraulic power steering system and only affects the electronic assist.

SERVICE:

Replace the Power Steering Unit (PSU), also known as Power Steering Control Module (PSCM), in a S80/V70/XC70 (P3) according to VIDA Removal, replacement, and installation under 6 Suspension and steering > 64 Steering > 645 power steering equipment > Actuator speed dependent servo assistance.

On XC90s and S60s, there is no replacement procedure in VIDA, so the location is shown in this document. Once the driver's kick panel is removed, the PSU is visible.

In the VIDA parts catalog, the PSU for S80/V70/XC70 (P3) can be found under 6 Suspension and steering > 64 Steering > 642 steering gear > Steering box ECPS.

In the VIDA parts catalog, the PSU for XC90/S60 (P2) can be found under 6 Suspension and steering > 64 Steering > Steering box LHD ECPS Speed-related power steering, LHD.

The way you verify you have speed sensitive steering is to look at the steering rack. If it has an electrical connector plugged in your vehicle has these feature. Whenever a fuse for the module blows or the module acts up the steering is almost inpossible to turn. Good luck to you and hope these information helps you out :)