I'll add to the other great suggestions.

- vacuum leak - there are a lot of vacuum lines on these cars and they are responsible for a number of different systems. There is a mess of vacuum and PCV lines running along the back (driver's side) of the head/block and that would be my first place to check, along with the 'elbow' on the front (pass) side of the intake (hard to reach - way down under the intake)

- transmission fluid - although it sounds like your problem is 'at speed', I was having a stalling problem when pulling to a stop, which seemed to go away with a trans fluid flush. I suspected the torque converter was failing to disengage the 'lockup' function when I was coming to a stop, and the engine would stall out like if you forget to disengage the clutch in a manual trans car. I was worried if that was my problem I was going to end up changing the solenoid(s), but it seems to be fine now (fingers crossed).

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1998 V70 AWD Turbo 190k+