Wattbike Power Output

I am using a Wattbike Atom with Zwift in Ergo mode for training workouts. I have the power calc set to 3 seconds however my power output is all over the place / up and down. I am really struggling with this aspect, I thought that the whole point of Ergo mode is that the app (Zwift) controls the power. if it says to work at say 140 watts for 5 mins whilst my cadence is static my power output fluctuates considerably. Any thoughts / help welcom.