That's the trouble with the MVBs you can't adjust the temperature and UVB exposure independently. They need to sit in those ranges to provide proper UVB. You can try setting up a low wattage ceramic heat emitter or halogen flood light on a dimmer so you can add a bit of heat next to it. Moving to the 160w might work too but it may produce too much heat. It's a bit of trial and error until it's dialed in but once you have it set, it should be pretty stable.