2001 Pontiac Grand Prix Poor/Looping Idle

To Randico: No, the engine has never (to my knowledge) overheated, at least not to the point of failure; 3.8s put out a lot of heat, and when I come to a stop on hot days, the temp goes up, but not to the point of hitting the redline (that issue I CAN fix myself; I need to check/clean the radiator fins after I get it back, anyway). I took it in for the aforementioned oil pressure problem this morning, so I can't do any further diagnostics until I get it back, but it did actually take all winter to heavily clog the catalytic converter; the reason I know that is because around the end of November 2013, I noticed it sometimes acted like there was a little bit of drag on the engine (slightly higher revs to achieve decent acceleration), so in March of this year, when it became bad enough to make it impossible to drive, I took it in, and they told me the cat was gone; I needed to get it replaced. Turns out, the cause, or so I was told at the time, was a vacuum leak around the intake manifold gasket, causing the air/fuel ratio to skew toward the rich side to compensate; they replaced the faulty gasket before I had the cat replacement done. When I went to the place for the cat replacement, they told me it was 50% blocked with carbon buildup (the tech who replaced it showed it to me, and said he'd actually seen them worse), and told me I needed to get it in for a tuneup as soon as possible. The AAA Car Care Center I called to schedule the tuneup told me to give it a week, and the rich mixture should even itself out, since the PCM needs time to readjust. That was 3 months ago, and, obviously, it's still doing the same thing it was back in March.

To Hmac300: I have only a limited amount of automotive expierence outside of lightbulb changeouts, oil and coolant checks, and A/C recharging, mostly brakes and door panels (with my father), but I do have a Haynes manual, so I have detailed instructions on how to take the engine apart, but not the experience to safely do so, or put it back together correctly (without help, anyway). I may have the place checking on it now do that.

Turns out a broken vacuum hose was leaking, causing an imbalance in the air/fuel mixture. AAA replaced the hose, and now it runs perfectly (as far as I can tell).