That is why I installed a new fan motor. The old one was worn out and did not circulate enough air. With the new motor, temperature regulates within + or - half a degree day in and day out anywhere inside the incubator.They're a good incubator but I've found that they don't hold the same temp throughout.
I also have it set up in my basement where temperature tends to be very stable. When the ambient temperature is stable, it is a lot easier to regulate an incubator.
The problem I had with styrofoam incubators was temperature fluctuations. From the outside edge to the middle of the incubator was never less than a 1 degree difference. That is enough to give erratic hatch results. I had chicks hatching sometimes over as much as 5 days from day 20 all the way to day 25. Are you sure you don't have problems with eggs taking too long too hatch?