Along with codes, are you seeing any Knock Retard (KR) at that range? Since KR will pull timing to fight the knock, a drastic reduction in timing would definitely be noticeable. Something else to look at. Since you say it drops off then comes back this would also sound like a possible knock issue.
If you're unfamiliar with it, KR works this way. When knock (...and it may not be true pre-ignition but we're going to lump all the possible events under knock since that's the way the ECM logic deals with them...) is detected, the ECM retards timing a certain percentage to get rid of the knock. This is called Knock Retard. Now, KR is a very eager beaver and the ECM tends to error on the side of caution (which isn't bad when you're talking about knock because it's one of the things that can throw a rod through a block which IS a really bad thing) and pulls more timing than is probably necessary to eliminate the knock.
Now KR will hang around for a while and slowly decay to make sure the knock conditions don't come back...so as you continue, the amount of KR applied and thus the amount of timing that is being pulled gets less and less until the KR is back at 0 and you're running the timing the tune is asking for. If knock comes back, the decay may be interrupted or the amount of KR increased depending on the severity of the knock.
So I would data log what is going on with your car as you drive and as you are experiencing this. Be sure to record boost and KR (along with RPM, speed, throttle position, timing, coolant temp, and current gear if you're an automatic) to see what's going on when it occurs. Also, if you're using a Bluetooth dongle and the Torque app (or Apple device equivalent) if the dongle is a cheap one it may not have the band width to data log all those parameters at a high enough rate to get a good log. I recommend either a more robust dongle or HP Tuners if you know someone that has it and is willing to log your car.
Last edited by Robotech; 06-24-2019 at 10:59 AM.