The first step would be to have the car diagnosed with a scan tool to verify exactly what is going on, watching TCC parameters, and also checking for codes. If lockup is in fact stuck on when it shouldnt be the car will lug around pretty bad and cause hard shifts. This is typically caused by debris clogging up the exhaust port on the TCC solenoid. Replacing the solenoid may make it work for a while but the problem will come back because that debris is coming from somewhere, and that somewhere is likely the lockup clutch inside the torque converter.
