GS400drive

i have 2000 GS400 tooked it to dealer, after all the trouble i went trough with dealer they can't fix my climate control for letting air coming out off the VENT atleast 5-10 degrees different across from left to right vents. even two center vents above climate control, the temperature outlet would discharge differenctly. this happen almost everytime after 20 or more minutes of driven... before 20 minutes it's would works perfectly. the climate control setting is on single stage. so i would like to hear from GS owner if there is any of you guys experience like this before another dealer tell me it's normal? thanks.