Hello, I developed a new problem with my 99.
It'll just completely shut down, I throw it in neutral and it starts right back up. The idle lowers a lot when coming to a stop, but doesn't stall at a light or a stop sign. The times it does it is mainly when I'm moving (yesterday it happened up a hill at about 35mph, and today getting off the highway at 45). It's pretty scary. I hope it isn't the ETM, I'm getting no surging idle when parked or in drive, nor is it hesitating. No CEL, however I do get P0172 that seems to come and go.
Thanks for any help in advance!

I have had this exact thing happen with my 99 XC twice in its 12 yrs. My money says it is time to replace the fuel pump relay [$35 at fcp groton] The symptoms will come and go as the relay slowly fails and then one day it will finally stall at 65mph on the Interstate as it did for my wife. Terrifying but an easy fix.

it sounds like your idle needs to be adjusted or you have a faulty O2 sensor

CEL went away, passed inspection and it hasn't stalled in 1200 miles, don't know what it was.

Your p0172 is the engine running too rich, usually caused by a vacuum leak. Look around the intake manifold for cracked elbows or loose vacuum lines. Check the electrical connector for the MAF at the rear of the air filter.

Klaus
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Always willing to listen, just not able to take direction.

Eeewww .... that's bad. Once I had a problem like this I believe. It turned out that there is a crankshaft position sensor whose wire is draped over the top of the transmission. Years of running the car had vibrated the insulation off of a portion of the wire leading to the sensor. The uninsulated portion of the wire used to touch the transmission case and short out, cutting the power to the motor. Then it would seemingly reset somewhere and start up again. I'm shooting in the dark here and this wire may be well secured on your car. But Volvo stories are many and this is one

Klaus, the MAF is new, so I will check for vacuum leaks.
What about the ECT? I read that can make it rich, and it doesn't run smoothly when first started for a few seconds (in the cold), and doesn't heat up as fast as it used to. The needle stays at one tick mark below the 3 o'clock line for a few minutes (thermostat is new). So is that something to start looking into?

As for the crankshaft sensor- did you end up replacing it? Or wrap it in electrical tape or something?