Toyota corolla 94 overdrive problem when car temprature low on 1994 Toyota Corolla

I have toyota corolla 94 auto transmission with 148000 km drives well exept when i drive car on freeway and car temprature goes down gear automatically change and rpm goes high. I change oil but situation still same. When temprature needle bit up gear shiting correctly

by in Wake Island, HI on May 24, 2013
1 answer 1 comment
ANSWER by on May 24, 2013
If the engine is not warming up to operating temp. and staying there , then possibly the thermostat is not functioning properly.
COMMENT by on June 02, 2013
Dear Zeptie You are champ car is fine now. you were right all mechanic advice me to change transmission but it was only thermostat. Thanks