1994 Park Avenue P0101 stalling, rough idle, loss of power

[jkmhb]

Symptoms: rough idle, stalling, severe lack of power. Sometimes runs ok for a while, but then runs bad again.
Randomly, but often throws a P0101 code (Mass Air Flow Sensor or circuit). Ran with the MAF unplugged, and swapped in another, no change.

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

So - what could cause a random vacuum leak that comes and goes as it pleases, and trip a P0101?

Thanks!