Have a 2002 V70 T-5 2.3

A mechanic friend has the same vehicle and said his did that and it 'was a valve by the breather' - cleaned it and worked OK for about a month., till he needed to do again ?

Any ideas what this valve is or could be?

Other causes to look for?

Check the connectors for coins pushed back.

see my post to - S80 2.5T - starts then dies; MAF?

PROBLEM SOLVED - REPLACED MAF SENSOR - ALL IS WELL. Thanks to all for your help - I learn a lot

As Derek said:

Rough idle or stalling
Idle hunting up and down when stopping at a traffic light
ETS light is on
ETM fuse keeps blowing
Limp-home mode
No communication with the ETM

All of the above could be caused by the Electronic Throttle Module. At least, unplug it, clean the connection, add dielectric grease, and then secure the connection with a twist tie.
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Keeping it running is better than buying new

OK thanks - in the shop now - starting by replacing the Mass Airflow Sensor and go from there.
The MAF is a BOSCH 0280218088 and all the inserts I see online are for Naturally Aspirated. Mine is a 2.3L Turbo.

Car starts and runs fine until it reaches operating temperature - THEN it starts acting up - idle wanders and slowly drops till it stalls. Restarts and repeats the same pattern.

If the car starts and runs well until the thermostat opens, check the engine coolant temp sensor. The ECT is close to the thermostat housing. A bad ECT will contribute to the problems you are having.

Easy enough to check. Unplug the wires(2), plug in a voltmeter and check the resistance. The colder the engine, the higher the resistance number. At thermostat open range, the ECT should read around 250 resistance.
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Keeping it running is better than buying new

Thanks - for $60 I ordered one - can I change it myself?

Any coolant loss?

$60 sounds like a bit much for a sensor.

Check this video:

https://www.youtube.com/watch?v=e3bASQM4EYs

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Keeping it running is better than buying new

Thanks - whats involved in changing it? DIY?

Before you purchase, clean the one you have, properly with the correct fluid. Don't use anything else, no carb or brake cleaner. Do it twice and let it dry out for half an hour before refitting. Clean the throttle body of carbon and gunk with carb cleaner. Best done with it off the car so that you can get to the back side properly. That side is usually dirtiest.
Don't forget all of the vac pipes I mentioned. There are a lot of them. Some go to a sort of tree below the MAF. They can crack and also pop off easily when you are pulling things about during checking.

Was he talking schematically or proximally? He could've been referring to the ETM as it is "by the breather". Even a hooter valve (boost control valve) is nearby but those just fail-you don't clean them.

Breather = air box? Valve = PCT valve just before the turbo?

When's the last time you inspected the air filter? Are you using synthetic engine oil? If you have more than 120K miles, does it pass the glove test?

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Keeping it running is better than buying new

Did you mean PCV Valve Klaus?

https://www.swedespeed.com/threads/ptc-nipple-cleaning.562338/

The turbos have a vacuum line that goes from the PCV to the intake, just before the turbo. If you use conventional oil, it will clog. The PTC has an electrical fitting.
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Keeping it running is better than buying new

Check all vacuum lines. Any service lights on the dash? ETS?