Interesting times!

I had to drive to the store to pick up a new clutch master cylinder for the Ford, so the Volvo needed to run.

I turn it on, and it runs normally for about 30 secs, then drops into a super low RPM mode; around 200RPM. I guess this is the "Limp-Home" mode. Previously, I had unplugged the AMM when the engine was running, and it returned to idle around 900rpm, so this had me scratching my head. Finally I noticed that the AMM was already unplugged, ok, engine off, plug it in, engine on, wait for it...

And the gremlin comes again! Finally, it drops down to zero, stalls, and I turn everything off. No luck. I thought maybe it would work enough for the short 6mile ride to AdvanceAuto, but now I'm in a bind.

Well, I've read a lot about bad connections... can't hurt to try some contact cleaner... so I unplugged the ECU and the O2 sensor, sprayed DeOx-it (Really good stuff!), let it dry off, plugged the connectors back together, and started the engine.

It runs... idles normally, and I wait for it to start seeking again. It doesn't, so I try to rev it up, and drop the revs.

Each time I release the accelerator, I hear a "Pop", I'm guessing it was backfiring into the intake. This happens two or three times, and I get concerned, so I disconnected the AMM from the air box to try to catch/feel/see it next time.

It doesn't happen again.

After abusing the engine through the throttle for about 10 more minutes, I decide to get to the store.

It drives just fine! I couldn't get it to stall (honestly, didn't try too hard; I just wanted to get home). But it ran quite well.

So... problem solved? I think not. I now have a pretty strong radiator leak, steam pouring out of the hood after a bit of driving, and coolant tank lost half it's contents over a 12 mile jaunt. But I guess that's another problem.

So, fingers crossed, hopefully it doesn't come back. I still have to clear the OBD codes to see if it really fixed the problem, or just postponed the inevitable, but I'm hopeful!

Thanks for your help. I'll definitely check my multimeters against each other.