Resistance during events

Every time I join an event everything will be running smoothly until at some point during the trainer will suddenly increase the resistance, making it really hard to keep watts and cadence up. It will do this for a short time and then release back to normal. If I continue the event it will do it multiple times.
During a normal workout or ride this never happens, only when in an event.

ERG mode is off.

I’ve got a Saris H3 connected to a Mac. Never had any other issues than this.

Thanks in advance for any feedback.

Did you try ant+

Thanks for the reply. Have not tried this, never used an ANT+ with a mac before, any suggestions on which might be best?