It seems the refractometer is the big issue here but it seems there is also an opportunity to improve your process with better control over mash temperatures. If you have an unintentional swing of around eight degrees that suggests not great control. The problem is if you're watching the thermometer you're seeing the temperature change at the thermometer only. Unless you have a recirculating mash it is likely some parts are cooler and some parts are warmer (especially when trying to warm up the mash). You may be getting cooler than you think in parts of the mash and too warm for the enzymes in others. I'm not sure what you use as a mash tun but it sounds like maybe a kettle because you're applying heat to raise the temperature. I'd try wrapping the kettle in a sleeping bag or thick blanket and see if that helps stabilize temperatures better.
