For me, DISabling Intel Turbo Boost was the only remedy possible. Had the exact same problem, tried external NIC, disabling C states, sleep modes etc. nothing else was effective..
I've fitted a top mounted LFP3 420 into the bequiet! 802 Silent case as well. Removed top fan tray with its rails, opened casing in top front to pull open just enough to pass AIO through into its compartment and padded the contacting spots btw AIO and casing with 5mm foam tape. Added 2 zip ties...