With the ETS light on I would already suspect a bad electronic throttle module if you have potentiameter code faults. What you can try to do to see if it helps is cleaning out the throttle body and plate and also the PCV system. Remove the fuse for the ETM in the engine compartment while doing this. When you have put everything back together install the fuse back on. Also, have you had recall 155 done? That was an ECM/ETM software update. Call Volvo to see if it has been done. Just give them your VIN# if it has been done, it still wouldn't hurt to see if there is a software update available for the ETM. After all this you still have a problem, you will need a new throttle which after replacement will need a software reload. At this point I would recommend you take it in to Volvo since there repairs now have a 2 year unlimited mileage warranty. Or you can look online to see who rebuilds them but make sure they can do your unit to avoid the software download?