Overdrive problem in Camry 94 V6 XLE

I have seen this problem occasionally. While driving on the highway the car will randomly downshift ( I have heard a dull thud sometimes ) and will never go back into overdrive again. The car's rpm is higher but otherwise it runs normally. Swithing on / off the overdrive switch makes no difference.After several hours the car starts driving normally again.

I had the mechanic check the car out and he could not find anything wrong with it.

Appreciate any thoughts on what the problem might be.

Thanks

--shiv