Have a 2002 V70 T5 - use 5-30 Castrol Edge Synthetic and 91 Octane fuel.

Intermittently the idle lowers and and engine misses and some times stalls at idle - then starts and runs fine - till the next time - thoughts to check out ? Air intake leaks? Air filter is clean.

Is an engine flush recommended - or will it cause more issues? Thanks.

Spark plugs? Perhaps a coil is ready to give up. Check for pending codes?

Flushing the engine is not needed if you are using synthetic oils, and I suggest that you not do it.

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Keeping it running is better than buying new

Any codes or CEL? just askin'.
Your vehicle's ~ 20 years old. Maybe change the fuel filter?

Have you owned this vehicle its whole life? If so, you'd know the oil change history. If oil is maintained faithfully, an engine flush is rarely needed. Another camp warns that engine flushes in neglected engines can loosen too much crud too quickly. The logic goes that big chunks of crud can block oil passages.

Yet another camp suggests using an oil with esters as they'll clean while driving.

Air intake is a good place to start. Inexpensive too! Check the large bellows type hose that connects to the throttle body. Common for them to get brittle and split or crack with age. Examine thoroughly. Also check the vacuum hoses condition and make sure all are properly connected.
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Will I buy another Volvo??? We'll see....