I know these arn't cheap but....
After rebuilding my old carb (that had stuck floats after a long winter) and increasing the run time to an hour or more, I still had poor idle and the sudden engine stoppage that was unexplained. I sprung for a new carb,as well as new racor and finishing filters, confirmed proper output pressure on the electronic fuel pump and changed ALL fuel lines and double clamped the darn things to reduce unwanted air intake. Don't forget to clean the intake screen too. Fresh fuell w/ additives to retard water and growth, and Yes, I do use a bit of MarvelMystery oil in the tank. My dad says to save the bucks and add a liter of kerosene to each 20 gal tank, but I prefer to spend $8 for the MM oil. My Bad.
Do the whole thing, all at once, and you may just find you can trust your A4 again! Plus spend more time sailing/motoring, and less time worrying what you need to do next to the old heap of iron.
I love my A4. It's the same kind of motor I've been working on since the 60's, it's basic, and if you put quality parts on it, the rewards are bountiful.
Don't fix it till you're broke, but fix it right the first time!