Elite zumo spins easier then harder than actual

I have an Elite Zumo paired with Zwift. It has worked fine but then i started noticing that the resistance within rides would vary greater than the climb/surface. It is now showing greater wattage/mph than I am actually doing- then it does a cycle of 5 pulses of about 5 seconds with greater resistance in each pulse- the final one is almost impossible to spin then it goes back to 2 or 3 minutes of greater wattage showing than I am actually doing.