usually at the parts store they ask if it's for the ecu, that would be the coolant temp sensor and that's not the part you want to try. coolant temp sending unit and temp switch are the same part. if you're already replaced it, #1 make sure it's the one for the car aka gauge cluster, NOT MOTOR, you're using. being that they're so easy to replace i'd try to warranty that part out, perhaps it was defective, and if it still does the same thing then the problem is either in the guage itself or there's a break in the wire. if there's a break in the wire you have 2 ways of fixing it. the professional way is to find the open and repair it, sometimes that's time consuming, sometimes it's easy to find. the other option is to run your own wire from the guage to the sending unit.