I have a 2005 XC70 with 145,000 miles on it. My AWD was not working at all until I replaced the stripped collar. Then I realized the AWD would only operate when I first started a drive with a cold vehicle. After a short time, sometimes just 5 minutes, the rear wheels stopped contributing on slippery surfaces. The AWD would always work again the next day for a short time. I replaced the temp/pressure sensor and the AWD SEEMED to work for a longer period of time (maybe 30 minutes) but then would become non functional again until the car was rested. The condition is very regular and predictable with no exceptions to the pattern. When I replaced the sensor the DEM looked very clean and the tiny drips of oil that came out from the sensor and valve appeared very clean. Is my issue a weak pump? What else could cause this intermittent problem?

Thanks for any advice or similar experiences you might have had..