i've had that problem since i got my car. i've eliminated the thermistor as the problem since i've replaced it. it could be the LCD unit that plugs behind the cluster. mine seems to be affected by temperature and humidity. when it's wet it starts to fade out. i was going to try and get at the LCD unit to clean the contacts but i had problems pulling the cluster out and haven't had another go at it.
let us know if you find anything out - this is one annoying gremlin. i'm not ready to spring for $200 to get a new LCD unit though...
1993 300e-2.8 - gone now <sigh>
"Do not adjust your mind, it's reality that's malfunctioning"