Rough idle out of the blue?

[GRSHOPR]

So I slowed down at a traffic light today, and suddenly my car started rough idling. It's a mostly stock tt. I checked out all my gauges, and my logger, which I luckily had with me at the time, and everything is good, except for my a/f ratio, which is so low, that the wideband gauge can't even register (I have to rev the engine to get it to drop below 18) and the rpm, is about 550 instead of the standard 550. The ISC is stepping up and down just fine, according to the logger, but it's really high, sitting about 24, instead of its usual 12-13. Is it just the O2 sensor? It's a jdm motor, so I only have the one off the rear precat. Thanks for your help.

[GRSHOPR]

Ok, so I got it home, popped the hood, just listened, since I was in a suit, having been at a wedding, and I heard a vacuum leak. Almost sounds like it's coming from the brake booster line. Is it possible for that to go bad? It sounds like it's coming from right where you attach the throttle cable to the plenum, but there is no other vacuum there.

[mb3000]

That line could have dryrotted over time. Need to do a visual inspection or a pressure test.

[GRSHOPR]

Visually the line looks fine. Is it possible for the valve to just blow?