It doesnt sound like a turbo problem. espically after 20,000 miles on a volvo rebuilt turbo. it could possibly be starving for fuel, i would change the fuel filter and drain the gas tank. put a fresh tank of premium in it...also the pre pump in the tank may not be working. i have seen those cause wierd problems. also definatly change the oil and filter. and again in a few weeks to get any crap out that might be in the oil. i had the same problem with the ACC on my 85 760...all the manuals i saw showed the temp sensor in a place where its not. on my car it was just under the head, on the drivers side in the back. i found the wire and grounded it. its actually a dealer recommended modification. its a white wire(i think) it comes out of the firewall around the heater hose area. i didnt even see the sensor untill i took the head off the car. it only alowes defroster operation ontill coolant reaches something like 95 degrees.
Mark