I've been working on this for a couple of months. It only happens when the engine is thoroughly hot. The car starts instantly and runs perfectly for the first 20 minutes, with full power acceleration (well, full power for an NA...). Typically it shows up when I've been driving at highway speed and hit heavy traffic. After about 5 minutes of crawling in traffic it starts having trouble finding idle, dropping down in the 300 range and wandering up and down. Sometimes it dies, but always starts right back up. If I use the gas pedal I can rev it up to about 1500 and keep it going. Most of the time once I get out of the stop and go it gets back to normal, but lately it's started cutting out at highway speeds.
I've thrown a couple parts at it so far. I've changed out the fuel pump relay for a brand new Volvo brand one with no effect. Also swapped out the IAC with a used one. About 5 years ago the OE fuel pump died and I put in a Walbro. I can't find my right angle schrader adapter so I haven't checked fuel pressure yet. I will, but the pump is quiet and there's been no change in WOT performance. Not getting a CEL, but probably still need to check for codes, trying to figure out the best reader option. It doesn't have the built in reader.
Not really sure where to go with this, any ideas would be greatly appreciated.