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
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1 answer 1 comment
ANSWER by , 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 , 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