and another thing that you might want to check is the motor on the back of the electric fan. make sure its not bad. on my gf's hyundai elantra 2 weeks ago her temp sensor went out and it had the same problem your explaining. the fans would turn on with the bad sensor but they wouldn't stay running. and id have to hit the motor or manually spin the fan to kick start it. so i ended up replacing the motor and the sensor and everything is kosher now.