Sudden loss of acceleration and power

This type of problem is very difficult to give advise online without actually checking the car out? The problem you are describing sounds like you are having an issue with the electronic throttle module. For the throttle to work properly you need good battery voltage going to the throttle. One thing you should really check when this happens is the charging system. There was some issues with the voltage regulator when it would get hot while driving for a while, the voltage regulator would just start to quit. If Voltage to the throttle goes low enough then the throttle stops responding. Most of the time there will be no messages on the cluster or warning lights but I would highly recommend you go to a shop that can actually scan all the control modules especially the central electronic module and the electronic throttle module for codes related to low battery voltage. If there is voltage codes then most likely the voltage regulator is failling. If you order one from Volvo, they stock an upgraded version of the regulator. Compare prices for the regulator vs. A complete alternator. If you are not saving much money, with the mileage on the car I would just replace the whole alternator. If there are different codes then further diagnosis would need to be done to see if it is a bad throttle, bad accelerator pedal, or an issue with the wiring? I hope this helps you with starting with some basic steps and please keep me posted.

Great addition to this thread! Please feel free to help out whenever you are on the site :)

Cheers, Ken

Check to make sure fuel pump pressure is within spec. Make sure there are no vacuum leaks. Check for tune up.

Check battery condition and charging system for proper voltage. Voltage drop battery cables. Check electronic throttle connections for proper conection, inspect pins for oxidation or damage. Chek for an open or short circuit from throttle to engine control module. If all looks good then you will need a new throttle. You may also need to have the new thottle loaded with software since it needs to be configured to the vehicle which only Volvo or a shop able to download software can do.

If you have a shudder during a gear shift then your transmission fluid is contaminated with coolant. You would need to have a glycol test done on the fluid. If it is contaminated then you will need to replace the transmission, radiator, and transmission lines.

Suggest have Alternator output checked

Has the check engine light come on? If it has can you post the codes on here to see whats going on? Also is these a turbo engine or non turbo? If these is a turbo engine and all of a sudden the turbo stopped working it will feel gutlas. If these is the case I would check all the vacuum lines to the turbo. They usually split open especially on the twin turbo. I would also look into the turbo control valve since they fail a lot as well. If these is a T5 inline 5 cylinder I would look at the rubber hose on top of the turbo for a split. Very common on the T5. Also look at the rubber charge air pipe to the cooler. But if the check engine light is on start with having it scanned first and posting the codes on here.

Inspect and test all the following listed below:

Oxygen sensor.
Catalytic converter.
Fuel injectors dirty/sticking.
Mass airflow sensor/Airflow meter.
Throttle position sensor.
Manifold absolute pressure sensor.
EGR Valve
Fuel pressure regulator leaking or defective fuel pump.
False air leakage.
Fuel contamination.
Foul/defective spark plugs.
Open spark plug wires.
Ignition coil/Coil packs defective.
Incorrect ignition timing.
Cap and rotor.

Note:If it doesn't apply disregard

Performance reduced warning message is almost like having the check engine light BUT this is actually for a problem relating to how it is going to effect the electronic throttle module so a regular OBD2 code reader will simply not pick up actual trouble codes then if you were to take it in to Volvo. Do you know what codes were happened to be scanned and tried to diagnose? If you have an issue with the electronic throttle you may still be covered by the 10 year/200k mile extended warranty but you will need to call Volvo to verify that it is. Also they would need to diagnose the vehicle and run some tests to verify that it is bad. Also when you call Volvo ask to see if campaign 155 has being completed yet on your vehicle. That was a software update for the electronic throttle and sometimes that will solve some issues with the throttle. But if the PCV system is dirty and the throttle plate is dirty as well, you will be responsible for paying diagnostic plus the repairs if you decide to have them do it. If you want to avoid going to the dealer then I suggest you find a shop that specializes with Volvo cars but you could be missing out on a free throttle repair which could cost you a minimum $1000. Hope this helps and keep me posted please.