would do a compression test see what you have, have seen a lot of cracked valves where once engine revs it sort of feels ok ish then under no load the crack dumps compression,, also injectors have a habit of going screwy or dirty,, i use seafaom and also add injection cleaner to 1/4 tank of fuel and drive it hard for a while also leave to soak as well help after,
seafoam
also seafoam can be added to oil 50 miles before change oil cleans out ie melts the carbon
also worth removing injector rail and flip it upwards and crank engine over can see them is any injectors are not atomising the fuel correctly.

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also worth removing injector rail and flip it upwards and crank engine over can see them is any injectors are not atomising the fuel correctly.