Cornish Boy

Help - I purchased a 2018 Forester XC Premium 2.0 diesel with 48k miles and full history in Nov 22. Until 4 months ago everything seemed ok (now has 66k miles). After a run on a dual carriageway for example as I decelerate the engine feels like it has a retarder operating on the engine and then when you accelerate again plumes of grey smoke emit from the exhaust. This will not occur if you rev the engine in neutral at this point but only when a gear is engaged and you accelerate. The engine then runs very "ragged". If you pull over, stop the engine for a few minutes and then start up again - all clear - until the next time !!!! I have had a diagnostic check at three different garages who say they can find no fault. After a few weeks of this the DPF light came on. DPF has been cleaned out and on running afterwards the diagnostic gave a clean bill of health to the DPF and confirmed that the EGR valve was opening and closing as it should. The analysis did show some minor oil dilution when the DPF was cleaned. Oil and filter has been changed. Car is not burning any oil and on a recent run from Cornwall to Cumbria and back via Oxfordshire it returned an average 58.9 to the gallon. I have taken it to three different garages of who all tell me they can't find anything wrong. Something clearly is - Any suggestions?