Well, a couple of things about vacuum leaks. Big ones (like it sounds you experienced with the inlet snorkel) will cause the idle to deteriorate badly. Small ones are less noticeable but typically result in poor off-idle performance; they aren't noticed as much at cruise speeds due to a wider throttle opening. So do give all your vac lines a good look-see, and don't forget the one to the brake booster AND your cruise control.
To check the O2 sensor you need a digital voltmeter, or even better, a scope. There's lots of info available on the web, but essentially you're looking for the sensor output to oscillate between certain high and low limits. Here's one link to get you started: http://www.ngk.com/learning-center/article/151/how-do-i-test-an-oxygen-sensors-performance-t
Just make sure your car is warmed up before you start testing the O2 sensor. You should find the connector at the top of the firewall on the passenger side - easily traced from the sensor itself.
If it turns out you need a new one, you can save about $50 by getting the same Bosch sensor for a '92 V8 Mustang; should be PN 13942 at Autozone. PN13953 is the same but with a longer wire. You may need to splice your old connector onto the new sensor, but that's worth $50 in my book.
Lastly, have you looked at your spark plugs to see if there's evidence of running rich? Given your car's age, the fuel pressure regulator could be getting tired or sprang a leak. In fact, when you check your vacuum lines, sniff the one going to the FPR to see if it smells like raw gas. A really bad FPR failure will make the car stop running quickly, but small leaks can happen (although they're rare).
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