Once the coil-packs become sufficiently aged, the failure of any single coil-pack seems to stress-out the remaining coil-packs, resulting in a cascade failure if you don't replace the coil-packs as a set. In the early phases of "coil-pack death", the faults can be intermittent in nature; however, once they start to "go bad", it's just a matter of time.
When looking for replacement coil-packs, please note that the brand of coil-packs can make a significant difference with regards to long-term coil-pack reliability and durability, so search the forum threads for advice on this matter. As a 3.0L owner, I've had excellent reliability with the OEM coil-packs; my car regularly gets 50K-70K miles from a set of OEM coil-packs, so it's only an infrequent pain-in-the-butt when I experience that first coil-pack misfire...
This condition is a know shortcoming of the Audi ignition system, but Audi has been very reluctant to take responsibility for addressing/correcting the situation, so most of us just keep a fresh set of coil-packs and the appropriate tools in the trunk... ;-)
* 2003 A4 3.0L Quattro (6-speed manual)
* sport package
* 18"x8" Moda MD5
* Continental ExtremeContact DWS (235/40ZR18)