Misfire on cylinder 6 when warm, followed by multiple misfires

[LouisD]

(Introduction: I'm a Lexus IS200 owner from The Netherlands with few technical skills) 

For a few weeks now, I've been encountering misfire issues.
Here's the pattern:

- Engine runs smoothly on a cold start. 
- When it warms up there are misfires on cylinder 6 (P0306),
- eventually followed by misfires on multiple cylinders (P0300, P0302, P0303) as well as the P0174 error code.

Mechanic replaced coilpack and spark plug for the 6 (without actually making sure it was broken), yet the problem persisted.
Then a compression test was run on the 6th cylinder and it passed.

Since I don't want mechanics keep replacing expensive parts without actually have found the cause of the problem, I'm hoping for anyone here to understand the nature of the problem and point me in the right direction to solve it.

Many thanks, Louis.