What is causing misfire?

There are a lot of reasons for a misfire. Is it random or specific to certain cylinders? Could be spark or fuel. Could be faulty sensors, coils, or wires, or clogged fuel injectors. You need diagnostic skills rather than just looking at the obvious.

An 04 Audi could be any number of things, first thing get the codes, if it shows a specific cylinder misfire then test for a vacuum leak on that particular cylinder intake runner, injector pattern and a dry and wet compression, if all that checks out then you have an engine problem, like a cracked cylinder head, valve problem, piston burnt.