High power resistance is inconsistent

I have a problem with high watt/ high power resistance being inconsistent. It feels normal at a easy pace but when I get out of the saddle to put max effort (say for a short max effort interval) I seem to break through initial resistance which feels spongy at the bottom of the pedal stroke then be at a wattage that feels higher that perceived effort. 

I’m using tacx training app and manually adjust power to the workout session I’ve planned, so not using Zwift. I’ve always done this and had 2 flywheel trainers that worked fine this way. Is this an issue with the neo not having a flywheel?

please help as it is pretty useless as it is!