First of all, it's a 94 Z28 A4 with roughly 136k on the clock. Not the healthiest motor, wasn't babied before I owned it.

While idling (in any gear.. park or not) I generally idle right around 1k on the tach, which is a bit high I think for an A4. Anyway, every now and then the idle surges up to 1200, down to as low as 400...almost stalling out, then suddenly jumps back up to anywhere between 600-1000 (on the tach). All the while, my oil pressure drops accordingly... as idle drops, so does oil pressure... below 20 (probably 16-20 psi) all the way down to maybe 8-10 PSI.

I should mention there are quite a few issues that could cause this. There's a small vacuum line below the TB that runs to the EGR solenoid, and it's broken. There's also a vacuum line coming out of the fuel line wire loom that's broken. The elbow doesn't seal well around the TB, and when I push the diaphragm on the EGR valve (when the car is off) I can hear air escaping (probably from the gasket of the EGR). Early next week i'll be installing longtubes and getting rid of all EGR components. Soon eough, I'll have a CAI that will take care of the leak around the TB.

What I don't understand, however, is the oil pressure and how it directly relates to my idle speed, what effect would a vacuum leak have on oil pressure?

There's a code 83 that is stored (SES isn't on) that says TCC circuit or something similar. All of my research shows reverse lockout, but that's for m6, not A4s. I notice a hard 1-2 shift ever now and then, could that be related to the TCC solenoid/code 83?

Hey I had a similar problem with my oil pressure. I guess the gauges or wiring tends to go. Check your oil pressure with a gauge. Also the oil sending unit sensors tend to go. Oil Problems Check here...

As for the improper idle and broken vacuums lines, I would say its because of the vacuums lines. If the engine gets too much airflow then it messes everything up at idle etc.

Fix vacuums lines and gauge oil pressure.

My Haynes manual says oil pressure should be around just below 9psi and I think at 4,500 rmp at 24-26 psi

When you need to check codes for your LT1, use Shoebox's code list. It has the meaning of the codes for both the M6 and A4. Looking at thaet, you will see it is a valid code for the A4:

http://shbox.com/1/Dtcs.htm

Certainly points to possible problems with the torque converter lockup, which could cause intermittent hard shifting.

Oil pressure is relative to oil pump speed/engine speed, the lower the rpm the lower the pressure and vice versa.
repair/replace all bad vac lines and your idle problem should go away. Good luck

Thanks for the input, I didn't even think about engine speed oil pressure being directly related... that makes perfect sense. As for the rough idle, I figured it was the vacuum lines, but didn't say how it related to oil pressure... now I do. Thanks a lot.

Injuneer, would that be the actual lockup inside the converter (meaning purchase new torque converter... or a stall )? I've heard of a TCC solenoid, but not specifically for fbodies, or would that be a mechanism located inside the torque converter. Also, I've got a Datamaster UNI file that's a few minutes long anywhere from idle to punching it up to 70 with one WOT run. Could you possibly take a look at it and tell me if there's anything else wrong? My long term fuel trims run upwards of 140 (sometimes up to 160) and a few other things are noticably odd. http://www.mediafire.com/?zmfzznwx3em

I know next to nothing about the A4's. Why not go to the Drivetrain forum, and ask the experts there, including several trans builders who are also Supporting Vendors? That's why we have the specialized forums.