ERG mode and Watts slipping all the time

Any advice on how to solve slipping watts on Zwift. On regular ride, steady cadence on the flat also issue on erg.
Running iphone 8 with Zwift and a Wahoo kickr Core (that had been working perfectly before.) All firmware and app updates are applied.
About a week ago, ERG mode will not hold steady watts - drops randomly by 25% and returns immediately. Feels like gears are slipping and then back to erg.
In non-erg mode, watts drop randomly, but feels worse than ERG mode because the ride is at a higher output. Feels like you slipped a chain or a gear!
Zwift says it is BLE interference, however, I am in basement as always with no new devices or changes in my setup. And device are in range by 3 feet for the bluetooth.
Wahoo have run their data test on their hardware and the Watts are smooth with no interference on same network, same wifi, same devices on bluetooth, apple phone etc….
Have forgot bluetooth devices, started from scratch, spun down, re-connected bluetooth etc. and still having issue. (Remember this had been working fine before.)
HELP! Any advice please, I like zwift but can’t ride like this.
P.s. Tried sufferfest and erg mode is smooth and steady on it with same setup.
Thanks Simon