After reading all of the posts I would reccommend that you check your engine wiring harness for degraded insulation. It's a prevalent problem with your year car. I have seen this symptom in previous Volvos of mine as well as reading about it from other Bricksters.

You can check the wires one of several ways. The easiest and most obvious is to check the wire's insulation at the connectors. If they are ok then you may need to either slice the loom on the harness to examine the wires individually in the loom better or use an ohm-meter with an extra wire to do continuity tests between the ecu and the engine compartment sensors and valves. You will need to disconnect the sensors and valves when doing this to isolate each wire. I would also get the wiring diagrams for this year car to compare and ensure that the wires aren't mixed up (namely the IAC and TPS connectors.)