thermostats tend to stick as they grow older and are slower to open and close. maybe your maintenance procedures aggravated it. my thermostat was making the temps swing from 78 - 100 (<100 only because i have the cooling mod), so i changed it. now it's pretty constant at 90 - 95 in summer heat, with A/C on and stop & go traffic.
1993 300e-2.8 - gone now <sigh>
"Do not adjust your mind, it's reality that's malfunctioning"