Symptoms: rough idle, stalling, severe lack of power. Sometimes runs ok for a while, but then runs bad again.

Consistently throws a P0101 code (Mass Air Flow Sensor or circuit). I bought some MAF sensor cleaner and it didn't help.

When the car is running particularly bad, you hear loud hissing through the air intake. Which is why I was suspecting the EGR valve stuck open (an air intake or vacuum leak can also throw the P0101 code I read). But when the car was running OK, I pulled the plug off the EGR and it still started to run bad after a few seconds.

Still - a MAF sensor for this car is $121 new (cheapest I could find) and EGR is $100, so I'm still hesitant to just changing parts.

Thanks!

Unplug the MAF, it will stumble and throw a SES light, but should smooth out. If it runs better unplugged, then the MAF is bad.

Unplugging the MAF did no good.

In a 30 mile trip, the car had 3 "episodes" where it ran really bad, If I put it in neutral and rev, I can usually get it to recover. It seems like it gets an intermittent vacuum leak for some reason.

Start looking over the vacuum lines. However, a small vacuum leak wont cause a lot of problems. A big leak will, like your EGR tube.

did some testing with a data logger - USB cable with EFI Live pro software.
MAF reads 4.4 - 4.8 g/* at idle, about 20 at 4000 rpm, but occasionally drops to 0 and stays there for a while. This is when the check engine light comes on.
Also - the o2 sensor when running OK jumps between 75-925 mv, but drops to 15-40 mv when it is running bad. Something wacky going on - the above, plus whatever is telling the IAC to maybe open too far? The IAC does track in and out according to the scan program.

Before this, changed all vacuum lines, and the PCV. Ran a vacuum test to check for a plugged cat - came up negative (meaning no plugged cat). Vacuum does drop when it'* running bad.
Maybe I'm getting a lean condition without a vacuum leak.

Any ideas? thanks.

Looks like the MAF has a problem. Tap on it, does the engine stumble?

I think the MAF is good. I've used 2 of them and still have the same problem.

Plus, the bad running occurs even when the MAF is unhooked.....

Maybe a 12v or ground issue?

Could be, or a bad wire. Start checking everything over.

When I probe the o2 sensor signal - high (purple wire BF9 on the blue plug on the PCM), it reads low (20 mv or even negative) when it is running bad, and around 800 mv when it is running good. Revving to 3500 RPM or higher gets it to start running good most of the time, but then runs bad most of the time at idle.

A data logger ( OBD 1.5 to USB laptop running EFI Live pro) says it is getting only 6 volts at idle, but I measure 13.6 volts at the PCM for all 3 12v connections. The ground checks out OK also.

Now it is also randomly chiming - slower than the chime when you leave the key in.

What in the world?

Fixed - it was the ECU