Ok,
I posted this on another volvo forum..something I hope will help someone down the road...
I have a 2004 2.9 S80 and for a long time I would get the occasional "reduced engine performance"
message and the car would go into limp mode. Sometimes I felt a tiny little bump or jolt right when this happened making me think there was some kind of a short in the system.
The volvo dealership said it was a faulty wiring harness connection and wanted around a grand to replace which I declined. It didnt happen for a long time after they checked it and then it started happening again about 6 months ago.
Luckily, turning the car off and waiting a few seconds and restarting it cleared the message and the car drove fine afterwards...for a while.
I tried removing and cleaning both the MAF and throttle module along with its connection. I still got the occasional problem and then about 4 months ago while the car was in an indy volvo shop I had them remove and clean connections for the ECM and TCM. I have not any problems since then.
I wish I could find the link to the forum I found this information on about cleaning the connections. I think the reason I didnt do it myself is that there is a special tool needed.