Hi stevie, thats probably a sign that the stat is faulty, ie more open than it should be, since at higher speeds the pump flow is higher too so the sender sees higher temps, and at low speed the engine is overcooling. Is your fan running constantly at the radiator? The car seems to detect stuck open stats or other water flow problems silently but turns the cooling fan on all the time for some weird reason, ive seen a lot of folks with that problem on here.
Mine is rock steady at 90 no matter what I do, and warms up extremely quickly from cold.
Even with a fairly large leak at the coolant flange on mine (another saga) it doesnt run hotter or cooler, just right.