Wow, that sounds very out of character for a Brinsea. I have two octagon Eco models with the autoturn cradle. When I start them up to set eggs, I let them run for a day to make sure the temperature is steady where it needs to be. That way I can make adjustments if necessary.
Where do you keep your incubator? For the temperature to spike like that, I'd have to wonder if it were environmental factors at play. I keep mine on the floor of my closet for incubating because it's a quiet, draft-free place not near an HVAC register and then I have a metal rack in my utility room where I have the hatcher set up at eye level. Both places are out of direct sunlight and seem to work well.
I see you're in Scotland. I've only dealt with the Brinsea dealer in Titusville, Florida since I'm in the US. Have you contacted your Brinsea dealer? Perhaps you have something wrong with the heating mechanism. When I contacted the FL location about what I thought was a broken fan, they were very helpful in trying to troubleshoot the problem. It turned out the last time I disassembled it to clean it, I'd overly tightened the screw that holds the thermometer in place so the thermometer tip was holding the fan blade in place.
I'm trying to think of what else might have gone wrong. Did you acquire the incubator new or second-hand? If you took it apart to clean it after a hatch, do you think some cleaning fluid may have gotten into the computerized area? I use a dampened paper towel to remove fluff and then q-tips for the finer work. Then I put the bristle attachment on my vacuum hose and carefully sweep the fan. I nearly pulled apart the red rubber-coated wire thing that runs around the top thinking it was a rubber band the first time I took apart my incubator to clean it.