Transformer Voltage Sag

one of the 18MVA 138kv/13.8kv FP transformer with 33 LTC Tap position, recently shows a voltage sag problem when the LTC tap changes. The voltage sag is for 8mS and it goes back to normal after 8ms. we take the DGA and open the LTC it looks like every thing normal. we performed a TTR also everything normal.

Is someone has some experience on this, what kind of test may help to identify this problem?

Thanks