6L80 shifting problem

Sorry in advance if this thread was posted in the wrong section as it is my 1st post, and forgive me for any typos...etc as English isn't my 1st language.So I have a 2010 C6 with the automatic transmission and ever since I got it there was always this weird transmission problem. Let's say I'm cruising at 50mph which would be on 6th, a normal car would downshift to 2nd and immediatly accelerate. My car doesn't do that. What would happen is that it'd go back to 2nd and rev the RPM to the correct spot but there's a delay in the acceleration for like 0.5 sec then there's a knock ! After the knock the car would accelerate. This symptom only happens when the car has to downshift a lot of gears and doesn't occur let's say from 6th or 5th or 4th to 3rd. The car shifts smoothly when driving normally.

Another symptom that rarely happens is that sometimes when cruising, the RPM would drop as if the clutch was pressed on a manual car then gets released after like 0.5 sec.

There is also a whine noise when the car shifts from 1st to 2nd.

Could all of these issues be caused by the converter going bad ? I suspect the converter because when driving normally the car shifts strong and doesn't slip. Also on WOT while using the correct gear for the speed, none of these symptoms occur.

 

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IDK about the possible converter issues but as far as the knock goes, are you using 93 octane fuel? You will get knock with crappy gas.