This is typically and extremely frequently caused by a failed inlet manifold gasket and usually where the manifold at number 1 or 6 cylinder meets the head. The fastening nuts at each end of the manifold loosen with time and a small piece of the gasket get burnt out so just tightening the loose nuts wont fix it. Use a metre or so length of windscreen washer sized hose with one end in or very close to you ear while you poke around the inlet manifold (and especially under the intake runners near the head) listening for a sucking noises of a vacuum leak. Do this with engine idling both when the engine is cold and once it's at normal operating temperature (as he change in temp can often open or close the gap where the leak is as the emtals in manifold and head expand and contract with changes in temperature). The usual old tricks of spraying oil, water, engine start aerosol, butane or similar to find the leak usually used on carburetored cars won't work on these engines as a) the leak is usually at the lowest point of the manifold gravity gets in the way and b) the ECU very rapidly adapts to small additions of these fluids and gases in the combustion mixture so no change in engine noise or revs will be noticable. You can replace it by undoing all the manifold nuts (including the one underneath that attached it to the block and pulling the manifold to one side and sliding the old gasket out and the new one in but it will be a challenge where having a trained octopus would be helpful. Watch out as the gasket can go in upside down and back to front; it will fit that way but will block the injectors spray so it won't run. You will also have to take off the upper intake manifold (fastened with torx screws) that has the variable runner mechanism to get at the nuts; the metal gasket on that is reusable with a thin coating of hylomar.
Note while a sticking throttle body butterfly is infrequently (unless a oiled after-market air cleaner has been fitted ) but nonetheless, sometimes the cause, it's also worth cleaning the throttle body butterfly with throttle body spray cleaner (e.g. ttps://www.supercheapauto.com.au/p/crc-crc-throttle-body-and-air-intake-cleaner---400g/358130.html or SCA Throttle Body & Carby Cleaner - 400g etc not old style carburetor cleaner) and a clean cloth per the instructions on the spray can in case it is sticking or grime on the butterfly is also contributing to the issue.