Tachometer going crazy and temperature light flashing

I was driving home and noticed my temperature light flashing (this has been going on for a while now). I recently had the thermometer changed, hoses changed, sensor changed, it just comes back, temperature gauge does not indicate overheating. So wasn't too concerned but then I noticed my tachometer all the way to the max (I was shifting appropriately). I checked today, I see no leaks, but still can't figure out why temperature light keeps flashing and tachometer is at max.