So, I will give you all some information about the issue I’m having with my Audi and bit of a timeline. It is a 2001 Audi Allroad 2.7 Bi-Turbo with 180K
A few months back I replaced the tensioner pully and serpentine belt.

I noticed after going on vacation while I was driving my car, it was not getting very much power going up a mountain road, and the car began to shutter/shake and the engine light began flashing. The vehicle has a bad torque convertor but I do not believe this is the source of the problem. The shaking would stop if I used tip-tronic and put the car in Higher RPM's. seemed like it wasn't getting enough fuel or something was wrong with the cylinders. I then noticed something wasn't right when I would be parked, in Idle, or especially if I was in reverse, the car was idling rough but if I gave it a tiny bit of gas it would stop. It has been cold and rainy here in Northern California, which could be a contributing factor but I always warm up the car.

After doing days of research, I got a code read done on the car and it was reading "Random Misfire" so no specific cylinder. I proceeded to change out all the spark plugs, I added a little oil to the car, and then bought a single COIL Pack and replaced one of them. I didn't have enough money to buy all 6 coils but I plan to. After doing this, the car now is idling a lot better, and now when I drive up the mountain road it’s not losing power or flashing engine light like before. I do however notice occasionally that the idle will be rough for a split second and the car will do a quick shake then idle normal.

Now lastly, I just noticed two days ago, if I turn on the Heater, Defroster, the Voltage gauge begins to fluctuate slightly and the car begins to idle roughly.

These are the things that I have decided may need fixing, or replacing. Let me know what you think and If I am missing something I'd really appreciate it.

-The Rest of the Coils
-The Fuel Pump
-The Fuel Filter
-The Alternator
-A Bad Pully for the AC?

 

First thing I would check would be the groundin points on the engine wiring harness and the ground straps connecting the engine to the body and the battery to the body. Also put a voltmeter on the battery when it's running. If at any point the voltage drops below 12 volts (have someone turn the heater, defroster, and other high demand electrical items on and off), you may have a bad voltage regulator. This part mounts to the back of the alternator. The brushes can wear down and not allow it to keep up with the electrical demand on the system. One of the firsts things you'll notice on a voltage drop is a misfire or rough running due to the high current demand on the fuel pump and coils. Normal constant voltage on the system should run between 12.2 and just under 14 volts (max).

A voltage regulator will be about $130-150... You might find one cheaper. OEM or the Dealer will probably cost your left arm...

 

Thanks for the TIP! Do you know where the ground straps connecting the engine to the body are on this vehicle? I put a Voltmeter on the battery, and had a friend turn on the heater, stereo, ran the windows, and turned on the headlights/highbeams. The Volts went from 13.6 down to about 12.3 and the demand began to make the car start to idle Rough....I am going to Autozone today to have them run a test on the Alternator itself. If it's bad then I will replace the Alternator or possibly just the Voltage Regulator. Today I will also check the grounding points and the ground straps if I can locate them. My next step will be to order the rest of the 5 coils, however I have a strange feeling it won't fix my issue.

 

I bet it's the voltage regulator on the alternator. Since you experienced a rough running situation when the voltage demand went up, it has to be the alternator not putting out enough juice to power everything. The electrical demand on automotive electrical systems is quite a bit these cars.. Don't spend the extra money on the coils until you get the alternator/voltage regulator fixed. You can remove the regulator while you have it out and check the brushes to see if they are worn down and not making good contact. When testing it, if there is a lot of visible arcing, they aren't making good contact so the current will not flow as well as it needs to.

Also since you've about isolated the issue, I don't think the ground straps have anything to do with it though. So I wouldn't waste time on them.