Resistance goes to zero when pushing hard

I have a Saris H2 operating on ANT+. Changed the belt just before xmas and no issues for 2 or 3 weeks and then find that when I stand on the pedals and push hard the resistance disappears and my turbo disconnects, soon as I ease up resistance returns and it reconnects.

I tightened the belt tension and this seemed to cure it but on todays ride it did it again. Belt tension was done to Saris manual pre-tighten, now its a little tighter.

Anybody got any ideas ?