I bought my 94 960 with about 153K miles in December 2017, have put about 16K miles on it since. I've had both ignition control modules replaced within less than 3 months of each other. The first one, the module for cylinders 4-6 had the output for cylinder 5 go bad, suddenly developed a miss at idle one day. Pulled the connectors to the fuel injectors one by one, pulling number 5 made no difference to the miss. Later after I got home I swapped the coil on cylinder 5 for a new coil, no change, put the original coil back. Note: pulled plug for cylinder 5 and it looked OK, a little dirty but cleaned up fine, not wet that I recall. Side note, I observed that the wiring to my coils was not very good; plastic connectors were breaking down. Wires themselves were not frayed or exposed just looked tired. Took it to my mechanic who found that one output was bad on the ignition coil for cylinders 4-6.

About that time I bought and later installed a new wiring harness for the coils from Dave Barton's web site. Super nice kit, highly recommend it. Spliced it in properly, insulated the wiring at the butt splices with heavy layer of electrical tape, covered the wiring harness under the top cover on the engine with wiring loom wrap for extra protection. I am an electronics tech by trade, just saying it was installed properly.

Now about 4-6 weeks after replacing the harness, a couple weekends ago all of a sudden it developed a bad miss at idle again, just like before. I pulled the fuel injector connectors ONE AT A TIME, this time the connectors on cylinders 1 through 3 made no difference at all to the roughness of the idle, and 4, 5 and 6 all made it idle even worse. I remembered my mechanic saying there were 2 power stages, each one for 3 of the cylinders. Turned out as expected that the other power stage for cylinders 1-3 was bad. My mechanic found this and replaced it.

It's running great again but my question is, how common is it for these power stages to go bad? As I said the second one failed perhaps 4-6 weeks after I replaced the wiring harness for the coils. Should I be concerned about it or is this common enough that this is no big deal for both of them to fail in the manner in which they did? I put in new plugs shortly after I bought the car, they are I believe NGK copper. I'm only guessing at the brand they could be Autolite copper but they are definitely copper plugs and not platinum. And the electrodes on the plugs look normal when I pull them.

Thanks.
Tom